Levels 4-11 and 4-12 have hidden unlock requirements, heavily inspired by the "anomalies" from Arcaea, and simply display as "0%" until these requirements are met. In reality, each of these "hell" levels are accessed through secret exists in the previous "heaven" levels. You also need to have beaten each of these levels once normally before their secret exits will appear!

If you would like to simply leave with some hints:

1. Cheat your way into heaven.
   
2. The beginning and end are the same.

For the full details, see below!

Simply finding the secret exit is not enough to unlock the anomaly level - you are thrown into it with a time limit (similar to challenge mode) you must clear the stage in to permanently unlock it.

The faster you reach the secret exit, the more time you will have within the anomaly! Every second saved is another second you will have to clear the level.

Because of this, however, there is also a time limit to reach the secret exit in the first place. Once you exceed this maximum time, the secret exit will vanish. This guarantees that you will have a reasonable amount of time within the anomaly, if you make it at all!

One important difference compared to challenge mode is that time travel will NOT pause the timer. Your only objective is to gather all the gems and get out!

If you fail to clear the anomaly (and you probably will), all is not lost - the unlock screen will now display 20% instead of 0%! Every failed attempt will add a further 20%, and once it reaches 100% (after five attempts), the level will permanently unlock automatically.

Note: Full spoilers ahead!

The secret exit here is simple: Go to the normal exit without having collected all the gems.

But wait, the gate doesn't open! As it turns out, only the bars of the gate are solid, and you can sneak in between them. You may need to wall-hit the base of the gate to get enough height for this.

With this in mind, the only obstacle you need to contend with is the time limit to access the anomaly. Being able to clear this stage quickly is the key!

The developer has a reference video for this on easy difficulty:

On easy difficulty, you have 2 minutes to reach the secret exit. Your time in the anomaly will be 3 minutes minus however long you spent in Heavensdoor.

On normal difficulty, the time limit is 2m30s, and you get 4 minutes in the anomaly minus your Heavensdoor time.

On hard difficulty, the time limit is 3m, and you get 5 minutes in the anomaly minus your Heavensdoor time.

Compared to Heavensdoor, this secret exit is MUCH harder to reach but with a far more lenient time limit.

The secret exit here appears at the very beginning of the level after collecting all the gems. However, the final gem is at the very end, and you can't backtrack from the end without a powerup! Time for some creative thinking - you will need to find a way to keep an extra powerup in storage to escape the end area.

Note that you can collect the same powerup multiple times.

Here is the developer's reference video for Easy:

In easy difficulty, you have 3 minutes to reach the secret exit, and your time in the anomaly will be 5 minutes minus the time spent getting there.

In normal difficulty your time limit is a very generous 5m, with the anomaly clock starting at 7m minus your time in Ouroboros..

Finally, in hard difficulty the time limit is 7 minutes with an anomaly time of 10 minutes minus your Ouroboros result.

By now, you should have all the knowledge you need to unlock hell and complete every level in the game. If you have any further questions, please use the comments below. Good luck!