The game is viewed from a first-person perspective. Each level requires the player to navigate interconnected rooms, collect weapons and ammunition, find colored keys, activate switches, survive enemy ambushes, and reach the exit.

The original release is organized into episodes of increasingly difficult maps. The marine begins with basic weapons and gains stronger firearms, armor, health, and power-ups. Secret areas reward exploration, while difficulty settings change enemy behavior and the amount of damage received.

Conserve powerful ammunition for groups of tough enemies, and use the shotgun or chaingun to handle common targets efficiently. Keep moving rather than fighting in open areas, because many enemies are dangerous when they can surround you.

Search walls, corners, and unusual dead ends for secret doors. Colored keys and switches often open routes elsewhere in the map, so remember inaccessible doors and revisit them after exploring.

Use enemy infighting to save ammunition: demons can damage one another when their attacks cross. Save before risky fights when possible, and learn the layout of levels with damaging floors or ambushes.