Duke Nukem 3D

Duke Nukem 3D is a first person shooter developed by 3D Realms and released by  Apogee in 1996. The story features the protagonist Duke Nukem fighting against an alien race. It contained many of the same features as Doom, although the atmosphere of the game is different: Duke Nukem contains humorous features, and also features pornographic material.

Technical
While Doom and Duke Nukem 3D feature similar technology, Duke Nukem 3D in some respects is technically superior. The rendering engine features slopes, overlapping and moving sectors, arbitrary scaling of textures and strong scripting capabilities. All of the game behavior is stored in external text files which can be modified to change almost any feature of the game. The registered version of the game came with an editor, Build, which included a 3D editor mode.

Similarities
Many of the monsters and other features in Duke Nukem are analogous to similar concepts in Doom. The following is a list of analogies that describe the similarities between Doom and Duke Nukem 3D (and other Duke Nukem games):

Trivia
A reference to Doom is included in Duke Nukem 3D. E1L3 contains a church. Standing in front of the cross at the front of the church, there is a switch. Pressing this gives a red tint to the room while the horizontal bar of the cross lowers until it appears to be fully inverted. Smashing the left stained glass window reveals a secret passageway. Walking down the passageway, a dead Doom marine can be seen. Duke says, "That's one doomed space marine".

The secret area was removed from later releases, possibly due to legal reasons.