Easter egg

Swastika
In version 1.0 of Doom, E1M4: Command Control level had a swastika hidden in it as homage to Wolfenstein 3D, but it was later removed.

NIN Reference
E4M1: Hell Beneath includes the letters "NIN" (or better NIИ with the cyrillic reversed N) in a part of the level, a reference to Nine Inch Nails.

Romero's head
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.

Heretic: Shadow of the Serpent Riders
Attempting to use some of the cheat codes from Doom and Doom II in Heretic has negative effects on the player:


 * IDDQD results in instant death.
 * IDKFA takes away all the player's weapons, except the staff.

Hexen: Beyond Heretic
The first episode's secret level Bright Crucible features a puzzle involving the heart of D'Sparil. D'Sparil was the final monster in Heretic.