The technology of DOOM included many profound advances and clever hacks, introducing to a wide audience the concept of playing over a network together ("co-operative mode") or against each other ("deathmatch mode") into the genre of first-person shooter games. It was not the first first-person shooter to do so, though; a game called MIDI Maze on the Atari ST did so in 1987, using the MIDI port built into the ST.
The software method used to present a 3D visual experience was clever, but sacrificed generality for speed. Levels were two-dimensional; and even though each section could have a different height, no part of the map could extend above or below another part. This compromise was made necessary by the fact that most computers were too slow to be able to handle a game engine with six degrees of freedom, as used in modern first person shooters, but careful level layout kept this constraint from being apparent to the player, giving the impression of an actual three-dimensional level.
These constraints also made the method unsuitable for generalized 3-dimensional applications such as CAD; they were, however, quite workable for a pre-built game environment, where the designers could dance around the limitations, and the commercial success of the game, as well as the abundance of third-party levels and add-ons that followed, demonstrated that these constraints did not detract from the gameplay experience.
