Easter egg

Ultimate Doom
E4M1 includes the letters "NIN" in a part of the level, a reference to Nine Inch Nails.

John Romero's Head
If you noclip behind the Icon Of Sin, you will see John Romero's head on stick. You can kill him to win.

Backwards Tongue
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".

SS Men
On map 31 SS Men from Wolfenstein 3D can be found, attacking the player.

Wolfenstein 3D
Map 31 looks like the Wolfenstein 3D level E1M1.

SS Men
On map 32 SS Men from Wolfenstein 3D can be found, attacking the player.

Wolfenstein 3D
Map 32 looks like the Wolfenstein 3D level E1M9.

Commander Keen
In the last room there are 4 Commander Keens hanging at a rope. You must kill them to be able to end the level.