While traversing the levels, the player must fight a variety of enemies, including demons and possessed undead humans, while managing supplies of ammunition, health, and armor. Enemies often appear in large groups, and the game features five difficulty levels which increase the quantity and damage done by enemies, with enemies respawning upon death and moving faster than normal on the hardest difficulty setting. The monsters have very simple behavior, consisting of either moving toward their opponent, or attacking by throwing fireballs, biting, and clawing. They will reactively fight each other if one monster inadvertently harms another, though most monsters are immune to attacks from their own kind. The environment can include pits of toxic waste, ceilings that lower and crush everything, and locked doors requiring a keycard or a remote switch. The player can find weapons and ammunition throughout the levels or can collect them from dead enemies, including a pistol, a chainsaw, a plasma rifle, and the BFG 9000. Power-ups include health or armor points, a mapping computer, partial invisibility, a safety suit against toxic waste, invulnerability, or a super-strong melee berserker status.