Trap

A trap can refer to any scenario where the player is led into a dangerous situation. They can be as simple as a damaging floor that the player must navigate, or a booby-trap that releases a group of monsters when a player throws an innocent-looking switch. Traps may be baited with a useful item or powerup, and are activated when the player walks up to the item to grab it. Almost any kind of trap is considered fair to a degree as long as it is beatable, and can add an element of surprise to an otherwise mundane level.

Most levels of Doom and other Doom engine games include some form of trap. The first trivial example of a trap appears on Doom's E1M1: Hangar, where the player must navigate a zigzag bridge over a pool of nukage.

Monster closet

A monster closet is a type of trap in which monsters are placed in a hidden room, where they cannot be seen or heard by the player. When the player activates a switch or steps over a linedef, a door disguised as a wall opens and releases the monsters, catching the player by surprise. The first example of a monster closet appears on Doom's E1M3: Toxin Refinery. When the player obtains the blue keycard, the lights shut off and a closet opens up, awakening the imps inside.

Teleport ambush

A teleport ambush is a type of trap in which monsters are stationed in a hidden room accompanied by a teleport that they will be able to walk on. The room is isolated on its own and the monsters are awakened prematurely by the sound of gunfire. When the player activates a switch or steps over a linedef, the monsters are released within the isolated room, and are able to step over the teleporter, summoning themselves at the teleport destinations assigned by the level designer. The first example of a teleport ambush appears in Doom's E1M9: Military Base, where the player steps on a red star to reach a rocket launcher and causes a series of monsters to appear in front of them, simulating the player disturbing a highly unstable demonic shrine.