Doom is played in first person, i.e. the player experiences the action from the character's point of view. The objective of the level is simply to locate the exit switch, which allows you to move on to the next level, surviving all hazards along the way. Obstacles include monsters, floors of lava or radioactive waste, falling ceilings that crush the player, and doors that require cards or "skull keys" to open and can be located nearby. Levels have mazes and secret areas with some reward for discovery, including exits to secret levels.
Doom has a simple arsenal. The player starts with a pistol and can use his fists if ammunition runs out, but better weapons can be obtained; these are: the chainsaw, a shotgun, a machine gun, a rocket launcher, a plasma rifle, and the BFG 9000. You can also take items such as a backpack that allows you to carry twice as much ammunition, armour, health kits, and spheres that give different powers such as partial invisibility, invulnerability and 100% health.
The fist is the player's stock melee attack. It is always available unless the player has picked up a chainsaw.
The chainsaw is a melee-based weapon that has appeared in every Doom title in the series.
The shotgun is one of the most versatile and useful weapons in the Doom player's arsenal, equipped in all classic Doom games.
The chaingun is a rapid-firing, multi-barrelled automatic weapon. It uses the same ammunition as the pistol, and is fed from the player's shared stock of bullets.
The pistol is the player's default weapon, and fires bullets. Each player enters the game with a pistol, fifty bullets, and their fists, with the pistol selected as the active weapon.
The rocket launcher is used to fire rockets, missiles that fly in a straight line and explode when they hit anything solid.
The Plasma Gun is a futuristic weapon with a barrel that roughly resembles an accordion, which fires blue and white bursts of plasma.
The BFG9000 appears as a bulky steel-grey gun which fires large balls of green plasma. For general purposes it can be considered the most powerful weapon in the game. The initials composing the weapon's name stand for "Big Fucking Gun."
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, using magic abilities, 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 Zombieman is a possessed and/or undead former soldier which has been turned into a zombie during the invasion from Hell.
The Shotgun Guy is a black-wearing human who has been turned into a zombie during the invasion from Hell. The monster appears as a red-eyed bald man wearing bloodstained, gray-black clothes, and wields a shotgun.
The Imp is a human-sized humanoid demon, the most common opponent encountered in id Software's IWADs, thus being considered the "backbone" of all hellspawn in the entire franchise.
The Demon is a monster found in all classic Doom games up to date.
Spectres are the partially invisible counterparts of Demons. Except for their blurry appearance, they are exactly the same in behavior and attributes and thus may be thought of as Demons who have a permanent partial invisibility power.
The Lost Soul was first introduced in Doom's second episode, The Shores of Hell, but can be seen as early as E1M1 on the PlayStation and Saturn ports of the game where they can be spawned by a Pain Elemental hidden in a secret corridor.
Cacodemons are common demons encountered in all classic Doom games: Doom, Doom II, and Final Doom. They are also the mascot of the first classic Doom game.
A pair of Barons, referred to internally by id Software as the "Bruiser Brothers", start as the bosses at the end of Knee-Deep in the Dead, the first episode of Doom. Barons also appear as regular enemies in the later episodes and in the sequels; coincidentally, they and their weaker counterparts, the Hell Knights, frequently appear in pairs.
Cyberdemon is perhaps the most formidable and powerful enemy in Doom. It is a tall, muscular, minotaur-like beast with two dark black/grey horns, a cybernetic right leg, a large rocket launcher mounted on its left arm, red wiring on its lower torso, as well as patches of metal and wires on its right arm.
The Spiderdemon (also known as the Spider Mastermind, as named in The Ultimate Doom, Doom II, Final Doom and the Depths of Doom Trilogy's manual) is the final boss of the first Doom, and appears again in the final episode of The Ultimate Doom, Thy Flesh Consumed. The PlayStation and Saturn versions of Doom in turn feature two Spiderdemons as the bosses of its final level, Redemption Denied on harder skill levels. The Spiderdemon has 3000 hit points and is one of the toughest creatures in the game.
Viewed from the top down, all Doom levels are actually two-dimensional, demonstrating one of the key limitations of the Doom engine: room-over-room is not possible. This limitation, however, has a silver lining: a "map mode" can be easily displayed, which represents the walls and the player's position, much like the first image to the right.
All of the walls in Doom are drawn vertically; it is because of this that it is not possible to properly look up and down. It is possible to perform a form of look up/down via "y-shearing", and many modern Doom source ports do this, as well as later games that use the engine, such as Heretic. Essentially this works by moving the horizon line up and down within the screen, in effect providing a "window" on a taller viewable area. By moving the window up and down, it is possible to give the illusion of looking up and down. However, this will distort the view the further up and down the player looks.
The system for drawing floors and ceilings ("flats") is less elegant than that used for the walls. Flats are drawn with a flood fill-like algorithm. Because of this, it is sometimes possible if a bad BSP builder is used to get "holes" where the floor or ceiling bleeds down to the edges of the screen, a visual error commonly referred to as a "slime trail". This is also the reason why if the player travels outside of the level using the noclip cheat the floors and ceilings will appear to stretch out from the level over the empty space.
The floor and ceiling are drawn as "visplanes". These represent horizontal runs of texture, from a floor or ceiling at a particular height, light level and texture. Each x position in the visplane has a particular vertical line of texture which is to be drawn.