Way of Life, Conclave, The Reaper's Due and Legacy of Rome – 4 mandatory anti-randomness DLCs for any culture and religion, of which Legacy of Rome would be the most important for expansive gameplay, as it provides access to retinues. Retinues are a standing profesional army that allows instant attacks right after declaring a war and is stronger than regular levies. After investing in buildings and having competent commanders, a retinue army can even win battles when it's hugely outnumbered. In my experience, you can even win battles where you are outnumbered 5:1.

For a full Romuva pagan experience you will need: Holy Fury (playable pagans, warrior lodges, reformation doctrines, eldership, legendary bloodlines, etc.), Charlemagne(769 start, custom kingdoms and empires), Sons of Abraham (holy orders for all pagans, not just Norse).

Old Gods is another DLC that allows to play as pagans, but it you have Holy Fury and don't plan to play as Norse pagan, I guess, you can do without it. It adds 867 bookmark, new mechanics for revolts and adventurers and is required to get the Jomsvikings (a Norse holy order).

Monks and Mystics is also a nice expansion, because non-Hellenic pagans have access only to the warrior lodges. With Monks and Mystics, pagans can also join the demon worshippers (The Cold Ones) and potentially The Hermetic Society(requires the Astrology doctrine). However, the demon worshippers are risky and, imo, cause more trouble than what they are worth, so I disable them in the settings. And, since you can create books outside The Hermetic Society and can now equip only 4 books at the same time, the Astrology doctrine has lost some of its appeal. Therefore, when playing as a pagan ruler, you don't miss much by not owning Monks and Mystics. It's hugely advantegous for Christians, though.

Jade Dragon is not mandatory, but useful for called invasions, boons, tributary states and forced vassalisation. Silk Road doesn't hurt either.

Horse Lords is not mandatory, but also allows access to Silk Road and provides access to different type of tributary wars that I personally have never used, but other players are quite fond of this feature.

De jure drift, assimilation and requirement and exclave independence:
- If you want to restrict bordergore, restrict these settings as much as Ironman mode allows;
- If you want to maximise titular king and emperor titles that you hold, set these parameters to maximum.

Custom realms: it's more expensive to create custom realms than the de jure ones, so there's no real advantage of using them. However, there's no disadvantage of using them, neither, so you might as well enable them. The reasons to use them is to have more titles, which means more prestige, and to create an empire in a situation, when you would not be able to claim enough de jure land to form a de jure empire. With de jure drift enabled, you can create a custom kingdom and a custom empire and then wait for all the de jure kingdoms/empires to drift into your custom realm, and after the assimilation is complete, you can create those kingdoms/empires as titural titles that no vassal of yours will desire. However, this approach can backfire when you reach your vassal limit and it's time to hand out viceroyalties, should you decide to use them instead of proper kingdoms. By assimilating kingdoms too early, you would contribute to bordergore.

Shattered retreat and army movement lock: turn off the retreat and enable the lock, unless you want to chase the enemy's armies around the map or you are afraid of defensive wars against stronger opponents (which is not very likely to happen, if you will follow this guide).

Raiding: "Historical", as it's a crucial way for you as a pagan to make money.

Vassal republics and theocracies "ON", because it's a good way to earn money.

Demesne size and vassal limit: as high as the Ironman mode allows, which is especially important for an expansive campaign.

Diplomatic range and matrilineal marriages: enabled, as it's important for collecting bloodlines and getting best councilors, spouses and commanders.

Pagan reformation: obviously, yes.

Religious conversion: as Romuvan you are already protected against other religions very well, and since it would be very hard to get rid of Old Romuva after the reformation with the doctrine setup that I will recommend, you want to set this option to "Default".

Devil worshippers and secret cults: unless you want more challenge, more societies and more reasons to get rid of your vassals, it's better to disable these options, as they also mean constant dealings with apostates (which might also be your family members) and greater chance of getting syphilis for no reason.

The Reaper's Due settings: I prefer to restrict minor epidemics and non-epidemic diseases and set Bubonic Plague to "Historical", as this kind of set-up is balanced enough to prevent mass extinctions, but also allows to test effectiveness of your hospitals.

Childhood focus alerts: as unrestricted, as much as you want to micromanage the education of your relatives and courtiers. However, there aren't that many pop-ups, even if you set this option to "All". Also, take into account that AI is bad at everything and that includes tutoring.

Way of Life settings: as I've mentioned above, AI is bad at everything and will cause a lot of mess. Therefore turn "AI seduction" and "Ai intrigue" off, but leave "Dueling" on.

Crusades are generally a good thing. They spice up the experience. Just make sure that you have at least 70k troops by 12th century (because of the Northern Crusade). It's not a challenging goal, so don't worry about it.

Jade Dragon settings settings: no practical reason to restrict casus belli and grace actions, however enabling major invasions might mean a more challenging gameplay, which might be a good thing in the late game, when you're almighty and powerful and things start to get boring and there's no real challenge left.

Horse Lords settings: "Unstable" makes the game easier.

Provincial revolt strength and frequency: "Default" and "Rare" (respectively) makes the game easier. They happen quite often, as AI is stupid at governing their counties. Therefore, I don't see any reason to make this minor annoyance any more troublesome than it should.

Mongol invasions and Turkic conquerors:: I prefer to set it to "Historical", as by 13th century you'll be strong enough to deal with them, if they decide to attack you. If you kept expanding to the east since 769, this event might not even happen.

Aztec invasion: initially I preferred to disable it, because I think it's a stupid idea and a cheap attempt to make the late game more challenging, however the late game really is quite boring, so even such a cheap challenge might be welcommed. So, I set it to ''13th century'', when you should be able to fight the Aztecs back.

All the other settings: as your heart desires and the Ironman mode with achievements allows.

Although, theoretically you can marry a Romuvan wife and convert to her religion or hire a Romuvan tutor and educate your children in his culture, it's easier to start as one of the Baltic rulers for a Romuva playthrough. Since Romuvan holy order Chosen of Perkunas is Lettigalian, it's easier for Lettigalian rulers to vassalize them. So, if you're not playing as a Lettigalian, you eventually might want to educate your children as Lettigalians or move your capital to one of the Lettigalian counties and convert to its culture. Survivability is incredibly important for the first generation of rulers, so you might wish for a ruler with a military education, since they have better health. Good health increases the chance that your children will have good health, too. Military rulers usually have high martial skill, which, in turn, increases the levy size, which is also crucial for early expansion. The allocation of traits is semi-random, but in my experience military competent rulers in 769 usually are one of these: High Chief of Prussia, High Chief of Courland or High Chief of Lithuania. Note, that only dukes and above have access to technological research and that Slavic AI rulers tends to snatch Marienburg, Chelmno, Lomza, Podlasie and Selija, if you don't do anything about it fast.
Below is a list of recommended Baltic rulers in 769 with their advantages and disadvantages.

High Chief of Lithuania:
Advantages: Lithuania is the biggest Baltic duchy with 5 counties (starts with 3), claim on Samogitia and Deltuva, relatively large starting levy, duke level title, all starting counties are protected by rivers, which can also be a disadvantage when attacking.
Additional, fun facts that have no strategical significance: (1)the historical first and only king of Lithuania, Mindaugas, was from here; (2) de jure capital of Kingdom of Lithuania is Vilnius.
Disadvantages: needs to adapt Lettigalian culture, only one coastal county in the default duchy and it needs to be claimed, has to compete with Chief of Samogitia and potentially his half-brother – High Chief of Courland.

High Chief of Prussia:
Advantages: usually decent traits, controls a holy site, relatively large levy, duke level title, central geographical location in the Wendish Empire, proximity to Poland, Pomerania and Rugen (a holy site), a loyal vassal, heir to Galindia and Chelmno.
Disadvantages: small starting demesne, has to compete with the vassal, needs to adapt Lettigalian culture.

High Chief of Courland:
Advantages:relatively large levy, heir to Samogitia and Deltuva that is governed by his half-brother who tends to die young, but also serves as a back-up, if the High Chief dies young without producing a heir, duke level title, Lettigalian culture, proximity to Estonia.
Disadvantages: small starting demesne, has to compete with his half-brother and potentially High Chief of Lithuania, peripherial location in the Wendish Empire.

High Chief of Yatvingia:
Advantages: relatively large levy, controls a holy site, relatively large starting demesne, proximity to Poland, Pomerania and Bryansk (a holy site), duke level title.
Disadvantages: no access to sea, needs to adapt Lettigalian culture, hostile neighbours with claims to his counties.
-----------------------------------------------
Slightly less recommended:

High Chief of Zemgale:
Advantages:duke level title, Lettigalian culture, younger than other Baltic rulers, good position to eliminate High Chief of Livonia (Suomenusko) and become a grand duke with control over Riga (a holy site), proximity to Estonia that is easy to conquer. Also, the HQ of "Chosen of Perkunas" are in Zemgale, therefore they get some of the building bonuses from the capital of Zemgale (Mežotne by default). It means that, if you vassalize them, they will get many of the same bonuses that your retinues get, as long as Zemgale is your capital county.
Disadvantages: small starting demesne, relatively small levy, but you can still manage, if you build training grounds and quickly conquer Selija, Livonia and some Estonian counties, peripherial location in the Wendish Empire.

Chief of Latgale:
Advantages: Lettigalian culture, relatively large levy, decent starting demesne size, proximity to Estonia.
Disadvantages: no access to sea, peripherial location in the Wendish Empire, not a duke (high chief), but it can be easily fixed by conquering or subjugating High Chief of Zemgale or establishing High Chiefdom of Latgale.

Chief of Galindia:
Advantages: controls a holy site, proximity to Poland, Pomerania and Rugen, heir to High Chiefdom of Prussia, central location in the Wendish Empire.
Disadvantages: needs to overthrow/assasinate High Chief of Prussia, needs to adapt Lettigalian culture.

Of course, everything that has been said about geographical locations is temporary, as you can expand, revoke titles and change capitals as you like. It's just that, in the first few generations, you are quite vulnerable, and might consider the path of least resistance.

Choose ammbition "Become the King of Lithuania" that provides you unlimited subjugation casus belli (CB) within the de jure kingdom.
This task is pretty straightforward – you acquire enough gold, piety and territories to become the King of Lithuania. Or, if you want, you can create a custom kingdom first. Unless you want to fight with High Chief of Polotsk, I recommend to subjugate Selija immediately. The Slavic rulers will also target Chelmno, Marienburg, Lomza and Podlasie, so you want to subjugate those rulers as fast as you can. I usually start with Selija and proceed clockwise from Latgale, encircling the eastern and southern borders of Lithuania, so that no Slavic ruler can invade the bordering counties. It doesn't always go smoothly, as other Baltic high chiefs start to expand as well and you might not always have enough units to subjugate whoever you like immediately. In that case, prioritize weaker rulers.
As you subjugate other Baltic rulers, they will not be happy with you holding their titles and might refuse a call to arms. To counter that, always keep your steward on "Build Legend", your diviner on "Build Zeal" and your Marshal on "Train Troops". On success, the steward will raise a warrior army (three stacks, each 7.5% of realm levies, minimum of 100 and maximum of 3000), which must be used in battle within one year (plus or minus three months) or else be disbanded, costing prestige; troops are automatically disbanded when war ends, although they will remain if another war was declared before ending the first one. On success, the diviner will raise a zealot army (three stacks, each 7.5% of realm levies, minimum of 100 and maximum of 3000; if not used against a ruler of another religion within one year (plus or minus three months) or explicitly disbanded, they will leave, costing piety). The marshal will increase your levy size and reinforcement rate.
Also, before going to war, check the army size of the targeted ruler and which of your vassals are likely to answer call to arms. Don't forget to build training grounds and warrior gathering grounds, whenever you can, and make sure that you always have some extra money (at least 50 gold should be enough at this early stage) when going to war (otherwise you risk negative province modifiers and reduced army morale). As you expand, you will be getting access to increasingly more competent commanders. So make sure to invite them to your court (from "Character Search" bar).
Note that if you don't have both (High) Chief of Courland and Chief of Samogitia as your vassals (you have one, but not the other), you risk losing territories. So, don't wait too long before subjugating them both.
After, you have subjugated all Baltic rulers and High Chief of Livonia, you have used up all the subjugation casus belli options provided by the "Become the King of Lithuania ambition". From there, you can establish the Kingdom of Lithuania and proceed by conquering Estonian, Polish or Russian counties or you can wait and get a different "Become the King" ambition, by moving your capital to a conquered county outside Lithuania. More details on this loophole here[forum.paradoxplaza.com].
Note that you get one subjugation casus belli per lifetime in addition to those provided by "Become the King" ambition. But you can use it only before using the "Become the King" subjugations. So, if you want to subjugate a non-Baltic duke/high chief, do it before subjugating a Baltic high chief.

You can postpone reformation until you become an emperor, but I don't really see any point of postponing reformation after that. After reformation, you will suffer attrition in pagan lands, but you can easily counter that by building forts and raising your military organisation to level 4 and beyond. So, it's not a big deal. You will lose the once-per-lifetime subjugation CB, but you will get holy war subjugation CB instead (unless you choose Peaceful nature), which has no lifetime restrictions. So, if it's your first ruler, proceed with the reformation; if it's your second ruler, use the subjugation CB against some large large duchy for the last time and then proceed with the reformation.

If you're the King of Lithuania and controlling 100% of Lithuania, you should already have control over 3 Baltic holy sites (Podlasie, Chelmno and Riga), which is the minimum requirement for reformation. You also need to have at least 750 piety and Romuva should have at least 50% moral authority. It should already have 30% from the holy sites. You should get the additional 20% by conquering counties and looting infidel temples within 20 years (each conquest and each temple desecration grants 1%). However, there are not that many temples around (they are most densely packed in Bohemia and Finland) that you can loot without any resistance. And, if you respect the peace deals, the 5 year cooldown also slows down your conquests. So, it's a good idea to fabricate a claim on Rugen and Bryansk as soon as you start the game. By controling one more holy site, you get guaranteed 40% and then it's easy to reach 50%. You shouldn't wait too long before pressing those claims, because otherwise Saxony, Pomerania or Bohemia will snatch Rugen, and nomads will snatch Bryansk. Usually, it's easier to take Rugen than Bryansk, but it depends on the situation on the map. It's possible to claim all 5 holy sites early, but it's hard to defend distant counties against raiders, if they are not connected to your main territory by land. Isolated counties also don't like to be isolated (increased risk of rebelion). Rugen is probably a better choice than Bryansk also because it's part of the Wendish Empire and Bryansk is not.
Note that, if you subjugated High Chief of Livonia instead of conquering his lands, you will have to revoke his titles and the title of the Shaman of Ledurga, if they are still Suomenusko. It's better to do that when you have unlocked and enabled religious revokation law, because then the Romuvan rulers will not get upset.

My recommended doctrines for Romuva:

1. Unyielding - +20% levy size. +30% garrison size; +80% Defense and +80% morale for all units if fighting at county of this religion; +40% garrison size at county of this religion; religion is resistant to proselytizing; great holy wars are available. Basically, "Unyielding" allows to keep the bonuses of unreformed Romuva and even improves them.
2. Ancestor Veneration - enables eldership succession, which you would lose otherwise, and ancestor veneration, which will provide you with a load of blessed bloodlines and make you popular with your dynasty, should you choose to become a religious head and distribute titles among your kin.
3. Sea Bound - allows to navigate through major rivers and significantly reduces maintenance of ships (down to 10%), which otherwise are very costly.
4. Temporal - because it's the only option that makes sense and it is required for "Ancestor Veneration" to work completely.

This setup has only advantages and no disadvantages. I don't recommend to choose "Defenders of Dievas", because "Ancestor Veneration" also enables eldership and, additionally, provides ancestor veneration, whereas resistance to proselytizing is already provided by "Unyielding", which is more than enough. Furthermore, resistance to proselytizing has also a downside - it will be hard to convert provinces of old Romuva to new Romuva.

A good alternative to "Unyielding" is "Unrelenting" + "Proselityzing". "Unrelenting" provides 10% attack bonus and allows to ignore attrition. However, it doesn't pay off in the long term, because you will increase your supply limit and resistance to attrition, by improving your military organisation. "Proselityzing" will allow to spread your religion faster and Old Romuva will no longer be a problem.
A good alternative to "Sea Bound" is "Astrology", which will provide you with Zodiac traits and access to The Hermetic Society. However, The Hermetic Society has been nerfed, since you can no longer equip more than 4 books at the same time. Besides, you can compose books even without The Hermetic Society.
If you don't care about eldership, then pick "Sea Bound" + "Daring", which will provide access to prepared invasions and viking traits. However, it's quite easy to incorporate "Blood of Lodbrok" into your dynasty, which also provides access to viking traits.

Therefore, all things considered, I really think that my setup is the best setup for Romuva.

Note: it's easier to convert provinces, if the diviner's culture matches the culture of the target province.

Chosen of Perkunas

If you have enabled Sons of Abraham, the Romuvan holy order Chosen of Perkunas will appear. If it doesn't happen immediately, wait a couple of years, or win a holy war. The order is weaker than the Christian orders, because it has only archers and light and heavy infantry, but if you vassalize it, you get accesss to 11 000 maintenance-free, reinforcing troops that you can use anywhere at any time (except for against other Romuvan rulers). You can vassalize the holy order, if the grand master (war chief) likes you a lot, has the same culture as you (Lettigalian by default), you are his de jure liege and you hold a title that is two ranks above his. So, basically, if you are a Lettigalian Romuvan emperor and give him some gold, he will become your vassal. In that case, you don't even need to give him a county.

Note: you have to make sure that the war chief of Chosen of Perkunas always likes you, otherwise he will leave your service.

This is another straightforward task - just keep expanding until you control enough de jure territory to form the Wendish Empire. Alternatively, you can create a custom empire, but it requires 3 kingdom titles, 1000 gold and 8000 prestige. You will be facing a lot of competition and opposition from Saxony, Bohemia and/or Pomerania (or, rarely, Polish dukes), which will be doing expansion of their own. It's both a good and a bad thing. It's bad, because Saxony is stronger than you at that point, and Bohemia or Pomerania might become stronger than you (mostly because they have levy cavalry earlier than you). It's good, because it reduces the chance that East Slavs and Avars (Pannonia) will expand into the Wendish Empire. Also, if you haven't reformed yet, subjugating a grand duchy is a bigger reward than subjugating a small duchy. In one of my playtroughs, Brandenburg subjugated all of Pomerania and half of Poland, and I subjugated Brandenburg with my second ruler, then revoked all the titles of Slavic rulers and distributed them among my kin. You should become an emperor with your second or third ruler, but the very least you should capture all of the east coast of the Baltic Sea before the start of The Viking Age (in 800) to prevent Norse rulers' expansion into the Wendish Empire and Estonia. It's a good idea to expand, by securing borders of the Wendish Empire first, so that neighbouring rulers can't expand into the Wendish territories. You might also wish to conquer Estonia, since it's such an easy target. Although it's not de jure part of the Wendish Empire, claiming it will give you nice, clean borders and will keep the Norse and Novgorod at a distance.

Although adopting feudalism temporarily reduces the number of available troops to you (you can't call to arms anymore), as an emperor you should be able to defend against any threats, especially if you have vassalized Chosen of Perkunas, raised about 10k retinue troops and used the "Unyielding" nature. Besides, AI is not that aggressive, and the only real defensive battles that you will have at that point will be against Norse and Slavic raiders.
Adopting feudalism will also weaken your vassals, because they will have less buildings in their demesne and will have to catch up with you. But you can help them out by using your prestige and piety to build buildings in any remaining tribal holdings.
To adopt feudalism you have to be reformed (or follow a reformed/organised religion), have max tribal organisation and have a stone hillfort in your capital holding. I recommend to adopt feudalism, when you have researched military organisation to level 4, as it's enough to raise more than 10 000 tribal retinue troops that you will be able to keep after the switch of governments and still maintain with prestige/piety. I recommend to use only prestige troops, because it's easier to get prestige than piety. And if you have too much piety troops, your piety will get negative and you will not be able to found kingdoms and start holy wars. Since the retinue types overlap in terms of their composition, I only use these three types of prestige retinues: 20 trapper units, 20 hunting parties, 20 veteran warrior units. It provides me with 6000 light infantrymen, 1000 archers, 1000 light cavalrymen and 1000 heavy infantrymen. That's 9000 soldiers in total, which is enough for any siege, but not too much to suffer attrition in most of the developed provinces. With level 4 military organisation, it also leaves me some cap space to hire feudal retinues of better quality with gold. Note: I don't hire Baltic warriors at the tribalism stage, because I don't have that much gold to maintain several units of them, since I spend it a lot for other things, and because veteran warriors provide me with heavy infantry anyway. Combined with the vassalised Chosen of Perkunas, you should have now a standing army of around 20 000 troops. So there's really no reason to postpone feudalism, especially for the following reasons:

• Feudalism allows to build hospitals, which protect you against disease outbreaks.
  
• Feudalism grants access to better laws, including "Imperial administration", which, in turn, grants access to "Regulated inheritance" (no passing of titles outside the realm) and "No internal wars" (less bordergore, improved economy, improved defence, vassals focus on external wars, which might expand your realm).
  
• Feudalism allows to build cities, which boost your economy.

Feudalism allows to create merchant republics, which boost your economy even more.
I recommend to use coastal provinces for merchant republics. As vassals they will provide you increasingly more gold. To create a merchant (burgher) vassal, grant a major a coastal county and then make him a duke. Good duchies for this purpose are Scalvia and Courland, because they are small and a part of your default Kingdom of Lithuania. It means that no other vassal king will have a claim on them. You can also use Prussia and Livonia, but I prefer to make them theocracies, if I don't intend to keep them for myself. Of course, it's good to hold some coastal counties yourself, as the grand majors will create trade posts in them that will provide you with gold, as well. Don't create merchant kingdoms, because prince mayors get too powerful and might hire several mercenary bands against you, which are a considerable threat in early stages of the game. It's fine to create merchant kingdoms in some backwater regions like Nenetsia, because they are underdeveloped and need some serious economic boost.

• Train your rulers as strategists or intrigants.
  
• When they are still heirs, make them commanders, so that your marshal can improve their martial skills and traits.
  
• Join the warrior lodge and reach the rank of a hero.
  
• Pick a warrior lodge specialisation, of which "Duelist" provides the greatest personal combat (PC) bonus.
  
• Duel in sparrings inside the warrior lodge to improve your dueling experience and PC skill.
  
• Antagonize weak characters until they become your rivals and duel them. But don't duel craven characters, because they will refuse duels. Don't duel old (>50 years), sick or handicapped characters, because you will dishonor yourself and some characters will dislike you for that.
  Note: there's still a small chance of dying, even if you duel a significantly weaker opponent. There's almost no chance of dying, if your PC >100, but it has happened to me. In my experience, the absolute safe zone is PC>200, but you can still get injured on legendary journeys.
  
• Don't duel and don't go to battles if you're sick or injured, unless you want to risk it.
  
• Don't duel for blood and don't go to battles, until you have ensured succession, unless you want to risk it.
  
• Get a good weapon. For Romuvans the greatest PC bonus (+16) is from the Axe of Perkunas, which one of the Baltic rulers possesses. It might be you or you can inherit it at some point. The second best option is any of the craftable tier 4 options, which require an emperor's rank, a lot of gold (3.5 x yearly income + extra 1000; min = 300, max = 3000) and prestige (at least 5000 has worked for me.)
  
• Get a good armor set. The best are the Angelic Guardian, Immortal and Green Armor. They all are tier 4 armor sets, which have the same requirements as the tier 4 weapons.
  Note: it's cheaper to craft tier 4 artifacts, when you are still tribal and your income is low. You can collect the gold by raiding and ransoming prisoners.
  
• There are some additional clothing (an arm band) and book artifacts that improve the PC skill.
  
• Build and improve hospitals to reduce your province resistance to disease outbreaks and survivability of your family and yourself.
  
• Hire good physicians with a high learning skill, theological education and following traits: "Renowned physician", "Genius", "Quick", "Erudite", "Diligent", "Ambitious" or "Scholar". Make sure that they like you a lot. I didn't find any official confirmation to this, but I think that it's better to avoid blind, one-handed, cruel, drunkard, possessed and lunatic physician candidates. Don't use the "Search for a court physician" option, because you can find better candidates yourself.
  
• Get a dog with the hunting focus.
  
• Whenever you get sick, stressed or depressed in a critical moment choose the family, theology or hunting focus.
  
• Carousing might remove "Stressed" and "Depressed".
  
• The theology focus might remove "Drunkard","Stressed", "Depressed", "Great Pox", "Blinded" and "Leper".
  
• Get good commanders and don't fight battles personally, when you're outnumbered.
  
• Collect bloodlines and integrate them into your family. "Blood of Lodbrok" will provide you with pirate traits and "Blood of Ashina"/"Blood of Attila" will allow your children to become mercenaries, which will provide them with "Adventurer" trait. Pirates and adventurers have better dueling skills.

Bloodlines improve the PC skill (most of them), give prestige and opinion boost and unlock new mechanics.
You can see the list of bloodlines in the last page of the ledger. You can see all the living bloodline holders by clicking on the blood drop icon on the bloodline founder's portrait. To collect bloodlines, invite the bloodline holders to your court. If they don't want to join your court, sell them a favor and ask to join your court as a return of that favor. They should agree, unless they are close relatives of their direct liege. Then marry them to your children and other kin or other courtiers. Give three concubines to male bloodlines holders, to ensure the chance of them having children. Marry women matrilinealy. Most of the patrilineal bloodlines can be passed down matrilineally. Matrilineal bloodlines can be inherited only matrilineally. To incorporate bloodlines into your close family, you have to marry your daughters matrilineally to bloodline holders. If the holder is of your dynasty, to avoid inbreeding, make sure that he/she is a distant relative, which is indicated by a greyed out blood drop and text "Your kinsman/kinswoman" (as opposed to other names, like "Your uncle", "Your nephew", "Your niece", etc.). It's generally safe to marry kinsmen whose closest common ancestor is their great grandfather/ great grandmother. If the holder is a woman of your dynasty, you can marry your sons matrilineally, as well. It's more practical to create paralel bloodline pools to combine several bloodlines at once. Mark them as "Characters of special interest" to keep track of them easily. Even if you're marrying people from different dynasties, it's better to check their family trees anyway. But even, if you made a mistake, it's not that big of a deal, because not all incestually born children get the "Inbred" trait.

• You will get warrior lodge bloodlines, by completing the legendary journey once in every 100 years.
  
• You will get blessed bloodlines, by venerating ancestors. A few centuries after the Reformation you will have more blessed bloodlines than you can keep track of, if you chose the "Ancestor Veneration" + "Temporal"/"Hierocratic" doctrine combination.
  
• You can forge a bloodline once per game (unless you switch to a ruler, who has no forged bloodline), by having 5000 prestige points and fulfilling the ambition "Forge a bloodline", which has different options and outcomes, depending on your actions. (See more details on wiki[ck2.paradoxwikis.com].)
  
• The only special event bloodline that is available to Romuvan rulers is the "Heir to Alexander", which requires at least 15 000 prestige points to trigger (also: a capital in Europe, Asia Minor, Jerusalem or Arabia; not content, emperor, adult, not in prison). Make sure to have at least additional 10 000 prestige points and at least 2000 gold in case it does trigger. It's a very good bloodline, because it unlocks invasion CB. More details here[forum.paradoxplaza.com].
  
  
• Theoretically, "Child of Destinty" and "Immortality" bloodlines are also available to you, but those events are very rare and it's even more rare to complete the immortality quest succesfully.

From historical bloodlines, pay special attention to these:

• "Blood of Niall 'of the Nine Hostages'" - +5% to light infantry and unlocks tanistry (which doesn't matter to you, if you have eldership).
  
• "Blood of Attila" or "Ashina Blood" - children can become mercenaries (solves the unlanded son problem and gives them the "Adventurer" trait (and, potentially, "Brave", "Diligent", "Cruel", "Wroth", "Venerious disease", "Great Pox", "Drunkard", so call them back after 5 years, if you consider them as your heirs).
  
• "Blood of Lodbrok" - unlocks pirate traits and events.

First, you should know that different kingdoms have different importance for different religions. (See wiki[ck2.paradoxwikis.com]). So, think well before you conquer Saxony, for example, – are you ready to defend it against Christendom?
However, note that you can still win Crusades even against overwhelming hordes, because they are disorganised and don't attack all together.
The fastest way to win a Crusade is to capture all Pope's holdings (not even all are required, just all Rome holdings). When you do that, the war score will reach 50% and he will offer peace. Just make sure that you don't lose any major battle during the war (major battle – a battle when two large armies face each other). And leave some troops to protect your capital.
To reach Rome as fast as possible, put Chosen of Perkunas and your retinues on the closest border to Rome shortly before the start date of the Crusade. When the Crusade starts, march your troops to Rome and start besieging the holdings.
Later in the game, all Crusades against you will be called off, because the Christians will not be able to muster more forces than you.

Some Crusades are scripted, including the Northern Crusade that targets European pagans. It starts before 1200. After it happens, a horde of around 70k Teutonic Knights and their allies will spawn in Luneburg (in Saxony, near Pomerania). You will be notified about the threat a few decades in advance.

First of all, don't marry title claimants outside your realm to minimise the chance of adventurers. Secondly, if you capture or plan to capture another territory, exterminate as much ruling dynasty members as possible or integrate them into your own. These two methods don't completely prevent adventurers from appearing, but they help.

If the adventurers do appear after all, here's what you can do:

1. Antagonize them and defeat them in a duel.
   
2. Assassinate them. Sending your spymaster and loyal concubines with high intrigue skill to the court where the adventurer resides helps. Leading troops personally in the province, where the adventurer is, also helps. But you can do that without suffering attrition only if they are not too far from your realm.
   Note: If the claimant is of your dynasty, you can get rid of the "Kinslayer" trait, if you have the "Silent murdered" bloodline. It also helps to get rid of the "Murderer" modifier.
   
3. If all of the above fails, or you don't want to kill your kin, just beat their army. It scales to your army's size, but never exceeds 30 000 men.

Retinues and The Chosen of Perkunas are going to be your main forces. Levies are going to be your reserve. There are many things to consider about your army and generals and you can go into great detail, analysing and calculating the best flank composition and price for value. But it's hard to remember all those aspects and, in the grand scheme of things, it doesn't make a huge difference, whether you have the absolutely best army possible or just one of the best – you are still going to destroy the AI troops. Following are a few basic things to know for optimizing your army:

1. The battles have three phases: skirmish, melee and pursuit (in that order). Different units excel at different phases and you can see their attack and defence values for each phase by hovering over them in your army window. Archers (and horse archers) excel at skirmish phase (well, infantry archers don't actually excel, they just are not that bad; you might say that they excel, if they trigger a good tactic, e.g. volley). Heavy infantry and pikemen excel at melee phase and pikemen are particularly good against cavalry. Light and heavy cavalry excel at pursuit phase (although, stat-wise HC is also good at melee and LC is also good at skirmish). Light infantry is just a back up for other units to absorb part of the damage. That's why what they lack in strength, they compensate in numbers.
   
2. You generally want your army to cover all three phases and it's a good idea to organise your army into specialised flanks so that one flanks consists mainly of cavalry, the second mainly of heavy infantry or pikemen and the third one mainly of archers. Perhaps, it's best to put cavalry in the centre, because then it can flank in any direction and deal more damage for all three enemy flanks, when they retreat. Flank specialisation is also important for triggering some good unit-specific tactics (e.g., shieldwall) and avoiding bad ones, because you mixed units too much. I prefer to give some backup for each flank to compensate for their phase weaknesses, for example, backing up defence retinue (mostly pikemen, some archers, cover melee and help skirmish) with some cavalry retinue (mostly light cavalry, some heavy cavalry, cover pursue and help melee and skirmish). Note that there four types of generic retinues that contain archers in different proportions (in some retinues archers are the main force and in some they are for backup, so there are multiple possible flank combinations). However, there are some units that are so good that you don't even need to cover all phases and some players just create armies where all units are of the same type. For example, Baltic warriors (heavy infantry) are not good for skirmish or pursue, but they can take so much damage, that excelling at just one phase (melee) allows them to withstand all the harassment in the skirmish phase and still do a lot of damage in the melee phase (especially with all the building upgrades and matching general traits). I however don't think it's fun and like to combine my heavy infantry with some pikemen. Another popular formation is pikemen or Scottish pikemen only. However, it has been argued that they still need around 20% heavy infantrymen to withstand damage and trigger the shieldwall tactic. As I've said in the grand scheme of things it doesn't matter, all those options are good, the only question is, which of them is optimal. There's no consensus on that.
   
3. The Rule of Two: a flank should contain NO MORE than TWO of the three major skirmish units (horse archers, archers, light cavalry), and no more than two of the three major melee units (heavy infantry, pikes, and knights). It's because of the reasons described in point 2 - to compensate weaknesses and to trigger good tactics and to trigger them consistently and most of the time.
   
4. Good commanders with a military skill of at least 16 (the higher, the better). Good commanders maximise the chance of a good tactic and minimise the chance of a bad tactic, even if they have bad traits that trigger them (e.g., craven, wroth). You can read which tactics are good or bad here[ck2.paradoxwikis.com]. You can read which commander traits are good or bad here[ck2.paradoxwikis.com]. You assign commanders to matching flanks (e.g., cavalry leader for cavalry). In general, you want at least one siege leader and at least one organiser in your army.
   
5. You can get access to different cultural retinues, by educating your heir in that culture. You can read about all the different cultural retinues here[ck2.paradoxwikis.com].

Note that you can split armies into smaller ones and make them follow the larger one by putting them together (not merging) and pressing the chain link icon on the smaller armies. This way you can keep the armies together for battles and split them for sieges to avoid attrition.

If you want to learn more details about retinue composition, read this thread[forum.paradoxplaza.com].

Additional resource: Navelist's combat tactics simulator[navelist89.github.io].

Generally, you want to adopt laws that expand your demesne and vassal limit. They are mutually exclusive, but I personally prioritize vassal limit in the early game, when I'm expanding and reaching the limit fast and don't have access to imperialism yet. This means leaving all the council privileges on. It's not as bad as it sounds, because there are always ways to workaround council resistance to some decisions (favors, changing councilors, making friends, temporary exceeding the demesne limit). Adopting imperialism requires absolute law (no council privileges), which means gradually expanding your demesne limit at the expense of the vassal limit. However, after imperial administration is adopted, you get +25 vassal limit, so it's worth it. Most importantly, imperial administration allows to restrict vassal wars.
You definitely want to restrict internal vassal wars, because they only increase border gore and weaken the vassals, making them vulnerable to raiders. So they are not developing their provinces. Don't worry - they don't become powerful enough to threaten you.
You might keep external vassal wars enabled, at least for a century or two, because then your vassals will expand the empire for you, even if you have a peace treaty with the targeted ruler yourself. You can then assist them indirectly with raiders. Just make sure to have law "Regulated inheritence" for all the kingdoms. However, external vassal wars also have disadvantages: they increase peripherial border gore, there's a risk that your direct vassals might become too strong, they antagonize the Pope (see GHW weight[ck2.paradoxwikis.com]), and they erode your religion's moral authority, because AI vassals are stupid and often start holy wars that they can't win.

If you have adopted "Ancestor Veneration" and had been giving away titles to your dynasty, it will ensure you popularity among your vassals. This boost can be enhanced, if you have some books in your possession that are dedicated to your dynasty. Sometimes, by venerating your dynasty members, you can get +100, +200 or +300 "very grateful" modifier with your kin within a life-time. It will allow you to get away with many unpopular decisions. The downside of granting titles to your kin is that you will have more title claimants among your vassals. However, it's a small price to pay for the stability eldership + ancestor veneration grant you.

With the current state of the game, viceroyalties are broken, so I'd say that they are not worth it. They don't work how they are supposed to. If you grant viceroyalties, non-viceroy vassals will dislike you for that, whereas viceroy vassals will start to grant out titles like candies within their viceroyalties or to revoke them, contributing to bordergore. There will be more frequent rebellions, which will drain your vassals' treasuries, which means that they will not be investing in buildings.

On the other hand, if you do use viceroyalties, you become very popular with whomever you grant the viceroyalty and you can repaint your direct vassal map as you see fit. Additionally, there's a chance that, when the kingdom or duchy title reverts back to you, you will inherit retinues with them, which sometimes could be of different culture that is not directly available to you.

Either way, it's easier to manage a few powerful vassals than many minor ones. I prefer to have one direct vassal per de jure empire (besides my default empire and my default kingdom). That way I have only one king-level vassal per empire who desires that empire and access to strong vassal armies anywhere on the map.

In general, no, unless you prefer primogeniture over eldership.
By adopting Chinese imperialism, you lose access to elective succession laws and you are stripped of all your extra non-titular emperor and all your non-titular king titles. You also lose all your titular king and titular emperor titles on succession, because they are inherited by your children. You also lose free title revocation from infidels/heretics. You can no longer declare great holy wars, but can still declare minor holy wars. You cannot create new kingdoms or new empires anymore.
On the bright side, you can have cities as capitals and holdings, which increase your income. You also no longer get opinion penalties for a different religion or a different government type. No one cares if you retract vassals (except for the vassal you retracted from) and you can freely revoke duchies (as with imperial administration).
For me personally, the disadvantages outweigh the advantages. The only thing I care about in Chinese imperialism is that you can put your dynasty (but not yourself) on China's throne. It's the hardest military achievement in the game and it would be like a cherry on top of the world conquest. Therefore, I'd adopt Chinese imperialism only in the last century of the campaign, if ever.

Little tips[forum.paradoxplaza.com]
How to become immortal?

Disclaimer This guide was written before the Great Works update (3.1.).