Things you must remember about Doom; its predecessor, Wolfenstein 3D, the first true FPS, sold a vast number of copies for the time – 100,000 by the end of 1993. A year and a half after Wolfenstein 3D's May, 1992 release, Doom sold a million straight out of the door, with a further nine million playing the free shareware version.
How did id, a four man team, manage this in a development window of less than one year?
A substantial chunk of the Doom design derives from its predecessor Wolfenstein 3D. But even as the team were working on Spear of Destiny, the underplayed prequel to Wolfenstein 3D, the far superior Doom engine was taking shape.
As the rest of the team - programmer-designer John Romero, artist Adrian Carmack (no relation) and designer Tom Hall - beavered away on Spear of Destiny, programmer John Carmack was starting to experiment with engine mods for Shadowcaster. Shadowcaster was , a gothic FPS/RPG from Raven Software that played like a first person Altered Beast. It ran at half the speed of Wolfenstein 3D, but added new technology: sloped floors, diminished lighting and walls of varied heights. Making new engines is the sort of thing Carmack does in his spare time – he's the engine programmer's Chuck Norris. He spent the latter half of 1992 conducting engine experiments before arriving, that December, at the architecture id would use for Doom.
His resulting engine absolutely flew; it was faster and smoother than anything on the market. The reason; Carmack took as many shortcuts as he could afford. While other developers had built true 3D worlds (the Ultima Underworld games being a notable forerunner), Carmack concentrated on speed. Doom levels are 2D maps that project upwards to form 3D structures.
Because of this design, players can play the game from the map mode (if they like a challenge). Zoom in close enough and you can even see individual rockets flying around the levels. The scale, variety of detail and speed of the game were the most important elements – Carmack even dropped a hi-color version because at 640 by 480 “it would have run at a pleasant 3 frames a second on the available hardware.”
The compromises didn’t matter: what mattered was that Carmack had created an engine that felt like it was from the future.
Having taken a short break in Disneyworld, the rest of the team had finished up work on Spear of Destiny in September 1992. Id also moved into a new headquarters that November, "the black cube" in Mesquite, TX. 20th Century Fox had briefly approached id about the possibility of making an Aliens shooters, but that idea ultimately proved a nonstarter. Nobody at id wanted to make a game where they would have to subject every decision to the approval of a major film studio. Another possibility suggested itself when John Carmack remarked that it would be great if their next game featured demons.
Tom Hall ran with the idea. While the rest of the team laid the technical groundwork for their next game, he created a Doom Bible. It was a new design document centering on the story of four characters fighting their way to hell and back over six episodes. (Co-op was originally at the heart of Doom and an early beta of Tom Hall's level - which became The Spawning Vats [e2m7] - has the starting room with the four marines around a card table when the aliens attack.) He created example levels, carefully researched from real military bases, and a hub system so players could revisit their past victories.