No monsters mode
From DoomWiki.org
No monsters mode is a mode that excludes monsters when playing Doom, Doom II, Final Doom and other Doom-based games. It is enabled by using the -nomonsters command line argument.
Bugs
Main article: Kill percentage flag used for -nomonsters and respawning
Several bugs are present with no monsters mode. On some levels, such as in the boss maps of Doom and in MAP32: Grosse of Doom II, monsters are a necessity for exiting. Also, the Icon of Sin's monster spawner is ignored by the -nomonsters parameter. When a level with a lone spawner and Romero's head is completed, each kill aside from the head adds 100% to the kill percentage.