Doom (typeset as DOOM in official documents and stylized as DooM in other media) is a 1993 first-person shooter (FPS) video game by id Software for MS-DOS. It is considered one of the most significant and influential titles in video game history, for having helped to pioneer, along with the 1992 Wolfenstein 3D, the now-ubiquitous FPS gameplay type, and has been frequently cited as one of the greatest video games of all time. The original game was divided into three nine-level episodes and was distributed via shareware and mail order. The Ultimate Doom, an updated version featuring a fourth episode, was released in 1995 and sold at retail. In Doom, players assume the role of an unnamed space marine, who became popularly known as "Doomguy", fighting his way through hordes of invading demons from Hell. With an entire one-third of the game (nine levels) distributed as shareware, Doomwas played by an estimated 15–20 million people within two years of its release, popularizing both the business model of online distribution and the FPS mode of gameplay, and spawning a gaming subculture. In addition to popularizing the FPS genre, it pioneered immersive 3D graphics, networked multiplayer gaming, and support for customized additions and modifications via packaged files in a data archive known as "WADs". As a sign of its effect on the industry, FPS games from the genre's boom in the 1990s, helped in no small part by the game's release, became known simply as "Doom clones". Its graphic violence, as well as hellish imagery, made it the subject of considerable controversy. 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 levels are presented in a 3D perspective, the enemies and objects are instead 2D sprites presented from several set 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 fight each other if one monster is accidentally harmed by another, though most monsters are not harmed by other monsters of the same kind. Levels can also include pits of toxic waste, ceilings that lower and crush anything below them, and locked doors which require a keycard, skull-shaped key device, or a remote switch to be opened. The player can find weapons and ammunition placed in the levels or can collect them from dead enemies; weapons include a pistol, a chainsaw, a plasma rifle, and the BFG 9000, among others. The levels also feature power-ups such as items that give health or armor points, increase the player character's maximum ammunition or health, fill out the automap, give partial invisibility, or allow the player to survive in toxic waste. There are also items which apply time-limited effects such as invulnerability or a berserker status. In addition to the main single-player game mode, Doom features two multiplayer modes playable over a local network: "cooperative", in which two to four players team up to play through the main game, and "deathmatch", in which two to four players play against each other. Online multiplayer was later made available a year after launch through the DWANGO service. Doom also contains cheat codes that allow the player to be invulnerable, obtain every weapon, be able to instantly kill every monster in a particular level, and several other abilities.