Doom is a first-person shooter presented with early 3D graphics. The player controls an unnamed space marine—later termed the Doomguy—through a series of levels set in military bases on the moons of Mars and in Hell. To finish a level, the player must traverse through the area to reach a marked exit room. Levels are grouped together into named episodes, with the final level focusing on a boss fight with a particularly difficult enemy. While the environment is presented in a 3D perspective, the enemies and objects are instead 2D sprites presented from several preset viewing angles, a technique sometimes referred to as 2.5D graphics. Levels are often labyrinthine, and a full screen automap is available which shows the areas explored to that point.
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.
The main campaign mode is single-player mode, in an episodic succession of missions. Two multiplayer modes are playable over a network: cooperative, in which two to four players team up to complete the main campaign,[5] and deathmatch, in which two to four players compete. Four-player online multiplayer mode via dialup was made available one year after launch through the DWANGO service.[6] Cheat codes give the player instant super powers including invulnerability, all weapons, and walking through walls.
