Easter egg

Swastika
In released versions of Doom prior to V1.2, E1M4: Command Control contained a swastika design on the floor of one room, presumably as homage to Wolfenstein 3D. Alpha versions of this level, dated April 2 and May 22, 1993, included the same room, but did not have the swastika design. The swastika was removed in V1.2 of Doom. The presence of a swastika would have caused Doom to be banned in Germany, if it had not already been banned for violence.

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

Romero's head
In MAP30: Icon of Sin, after the player teleports into the huge room with the Icon of Sin, a strange, unintelligible noise is heard. If the sound is reversed, it is revealed to say, "To win the game, you must kill me, John Romero". If the noclip cheat code is used to look behind the Icon of Sin, John Romero's severed head on a stick can be seen. Killing Romero ends the level.

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. In MAP31, SS Nazis take the place of soldiers and Demons are found in place of the dogs.

A Cyberdemon is in the place of Hans Grosse in MAP32; the name of this map also comes from Hans Grosse's name.

In the last room in MAP32 there are four Commander Keens hanging on ropes. They must be killed in order to exit the level.

At the beginning of MAP32, the message "You'd better blaze through this one!" is displayed; Billy Blaze is Commander Keen's real name.

Final Doom
Some places in the TNT episode, there's a wall texture: It's a message written in blood. It says Carmack(or Romero). This is referring to the creators of Doom, John Carmack and John Romero.

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 hub's secret level Bright Crucible features a puzzle involving the Heart of D'Sparil. D'Sparil was the final monster in Heretic.