Gameplay
Being a first-person shooter, Doom is experienced through the eyes of the main character. This character is not named throughout the game. The game's designer, John Romero, has pointed out that this is so the player feels more involved in the game: "There was never a name for the DOOM marine because it's supposed to be YOU." At its core, the gameplay is similar to classic shooter games (such as Space Invaders), presenting the player with the challenge of surviving while shooting every enemy in sight, but with its pseudo-3D first-person perspective giving environments a spatial representation that has a major effect on the level design and gameplay experience.
The objective of each level is simply to locate the exit room that leads to the next area, marked with an exit sign and/or a special kind of door, while surviving all hazards on the way. Among the obstacles are demonic monsters, pits of toxic or radioactive slime, ceilings that lower and crush the player, and locked doors for which a keycard, skull-shaped key device, or remote switch must be located. The levels are sometimes labyrinthine and feature plenty of items such as additional ammo, health increases and other "power-ups" along the way, as well as the occasional secret areas which are not immediately obvious as a reward for players who explore more carefully. To ease navigation through the levels, a full screen automap is available and shows the areas explored to that point.
Doom is notable for the weapons arsenal available to the marine, which became prototypical for first-person shooters. The player character starts armed only with a pistol, and brass-knuckled fists in case the ammunition runs out, but larger weapons can be picked up: these are a chainsaw, a shotgun, a chaingun, a rocket launcher, a plasma rifle, and finally the immensely powerful BFG 9000. There is a wide array of power-ups, such as a backpack that increases the player character's ammunition-carrying capacity, armor, first aid kits to restore health, the berserk pack (a dark first aid box that puts the character into berserk mode, allowing him to deal out rocket launcher-level damage with his fists and potentially splattering former humans and imps, as well as setting the user's health to 100% if it were lower), supernatural blue orbs (named soul spheres in the manuals) that boost the player character's health up to a maximum of 200%, nightvision, computer maps (which show every area of the level), partial invisibility, and protective suits that allow the player to survive in toxic acids.
The enemy monsters in Doom make up the central gameplay element. The player character faces them in large numbers, with the number generally increased when the higher of the game's five difficulty levels is chosen when starting a new game. There are 10 types of monsters, including possessed humans as well as specifically hellish monsters, all which vary in many ways. The monsters have very simple behavior, consisting of either walking toward their opponent, or attacking by throwing fireballs, biting, and scratching. They will fight each other if one monster is accidentally harmed by another (though most monsters are not harmed by the ranged attacks of their own kind).
Many versions of Doom (and its sequels) include secret levels which are accessed by the player discovering alternate exits, often hidden behind secret doors or in areas which are difficult to reach. In some versions of Doom II, both of these secret levels incorporate level design and characters from Doom's precursor, Wolfenstein 3D, which was also developed by id.
Aside from the single player game mode, Doom features two multiplayer modes playable over a network: "cooperative", in which two to four players team up, and "deathmatch", in which two to four players play against each other.
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