No, I did not find this Secret World o: If I had to guess, does it has something to do with all the random places with items, like the Chest and Chestnut Tree?
Viewing post in Chesscourt Mission jam comments
Actually no, those are just little easter eggs. Level 21 (the end screen) is secretly a playable level, and can be beaten. Look closely at the edges of the screen. (You can also use the press M, I & Z on the title screen to cheat any unlock all levels, including the secret world)
It's super difficult, so I didn't want to make it feel like a necessary part of the game. But if you're interested... there is more!