It's open.

Those who explore the Game in its entirety can access Stage 12 now.
I added everything I had in mind, though I will add more stuff here and there
If you find a "Special Door", there is now a low chance to enter Stage 12.
The Doors are well hidden and if you want to find them, you need to enter secret areas of the game.
You may need a few attempts.
Stage 12 is different. There is Danger.