Easter egg

E1M4
E1M4 description

Swastika
In version 1.0, this level had a swastika hidden in it as homage to Wolfenstein 3D, but it was later removed.

E4M1
E4M1 description

NIN Reference
This level includes the letters "NIN" (or better NIИ with the cyrillic reversed N) in a part of the level, a reference to Nine Inch Nails.

Map 30
In MAP30: Icon of Sin, when you teleport into the huge room with the Icon Of Sin, you hear a weird, unrecognizable noise. If you play the noise backward, it says "To win the game you must kill me, John Romero". If you noclip behind the Icon Of Sin, you will see John Romero's head on stick. You can kill him to win. History behind these two eggs

Secret levels
The secret maps, MAP31: Wolfenstein and MAP32: Grosse, resemble E1M1 and E1M9 from Wolfenstein 3D. Both levels include Wolfenstein textures and SS Nazis that attack the player. SS Nazis are in the place of soldiers and in MAP31 Demons are found in place of the dogs. A Cyberdemon is in the place of Hans Grosse in MAP32. In the last room in MAP32 there are 4 Commander Keens hanging at a rope. You must kill them to be able to end the level.