Thronefall manages to find a sweet spot between RTS and TD genres, a quite lucky find I must say. Focusing on "fun" and "simplicity":

Your character is your avatar in the battleground, a physical entity that you need to control and lead the battle by being there while surviving. It's quite fun to control and move around your character; responsive, fast enough and again simple. Auto-attacks allow you to focus on your positioning while also commanding your units doesn't overwhelm you.

Minimalistic art style doesn't clutter your screen even with dozens of enemies and units in the same screen. A small palette size creates more atmospheric environments, yet still every level manages to give totally different vibes from forests to deserts to tundra (I mean seriously just check the store page images). Also it's kinda cute. Yet still manages to create cool day/night transitions with the lighting.

I personally love progressions on any game, especially when it's done right... and that's exactly the case here. You can always feel the progression of getting better, from the core loop to the meta. As usual, you gradually unlock new features and upgrades as you finish the levels. On the other hand, you're also always starting from scratch in each level like a roguelike progression. You slowly unlock new building as you build your kingdom even when you have every upgrade unlocked (some shortcuts exist). So it's not overwhelming you as providing all the buildings unlocked from the beginning would. You're reaching your endgame goals by progressing throughout the waves and adapting along the way. Again a simple design yet powerful.

Another aspect that I appreciated was having premade building locations. I'm sure that many developers would've decided to implement free building placement in such a game, maybe even adding a soft-grid system to help out infinite placement possibilities. Yet such features would only complicate the gameplay and likely to steal away the fun part from the gameplay imo. I'm sure some would be disappointed for not having the full sandbox building environment but that wasn't the intention with this game. Simplicity is the key still. Maybe you're not deciding on the positions of the buildings, yet you're still prioritising which buildings to spend your resources on. Which is greatly countered by the economic vs defensive building options. While defensive buildings like towers would help you immediately in the current wave, not focusing on your income buildings would cause you to not being able to catch up with the increasing challenges. On the other hand, you'll quickly realise that income buildings are quite fragile without proper defences and doesn't provide any bonuses if they cannot survive the night. Hence it's all about adaptability and balance. High risk (now), high reward (in the later stages).

Although it seems all "simple" in the game, personally I find it quite challenging at times. Even though you can breeze through the levels just by reaching the end however you wish to play, extra challenges and quests could immediately ramp up the difficulty. Sometimes I played a level more than six times just to complete multiple quests at the same time. Especially when probably could've easily beat each quest separately in six different games. But hey, it's still great that you can create your own difficulty curve yourself...

Also this simplicity doesn't mean that the game goes pretty monotone throughout the campaign. There are at least 3 major build focuses for tackling the levels. You can focus on your own characters attack to brush off the enemy waves, be a leader to your units while they do the heavy lifting for you (a bit more historical approach) or turn the game into a literal tower defence. Not counting the economical focuses here. There are many combinations that you can pull among them, depending on your play style or preferences. However, trying to take on challenges kinda forces you to try out different styles and get out of you comfort zone every now and then.

So overall it's a great indie game that you should definitely check out. Amazing work Jonas and Paul! You're an inspiration to all the other indie developers out there. (You can also follow the dev logs of the game on their youtube channel)