Wolfenstein
Silas Warner, an American programmer and an employee of software company Muse Software, was a pioneer of early video game development, especially the stealth genre. He had access to computers in college, and when he worked for the IBM Mainframe architectures before joining Muse Software. The computer he used for game-development, P.L.A.T.O., was one of the firsts with multiple functioning terminals and an example of early computer networks. Warner developed, with P.L.A.T.O., a series of classic video games like his flight simulator and arcade shooter. Inspired by the arcade shooter video game "Berzerk" and the 1961 war film "Guns of Navarone", he developed "Castle Wolfenstein", a stealth-adventure video game in which the player controls an unnamed American prisoner of war as he steals German files containing secret war plans, while avoiding, disarming or at times killing hostile guards and trying to escape the fortified Nazi stronghold 'Castle Wolfenstein', set in World War II. The game was published in 1981 by Warner's company M.U.S.E. Inc. (Muse Software).
Wolfenstein 3D
Ideas from the Deep, now formally established as id Software, used some of these to prototype ideas for their own games. Adrian Carmack used them to push his preferred, dark art style, while John Carmack began to experiment with 3D computer graphics, which until then was largely the purview of flight simulation games such as "Wing Commander" (1990). Carmack found that this was largely due to the limitations of personal computers of the time, which had difficulty displaying a fast action game in 3D due to the number of surfaces it needed to calculate, but felt that the increasing computational power of PCs meant that it may be possible. During 1991, he experimented with limiting the possible surfaces the computer needed to display, creating game levels with walls designed only on a flat grid rather than with arbitrary shapes or angles. He also took the unusual approach of creating the displayed graphics through ray casting, in which only the surfaces visible to the player were calculated rather than the entire area surrounding the player. After six weeks of development, Carmack had created a rudimentary 3D game engine that used animated 2D sprites for enemies. Id Software then used the engine for the April 1991 Softdisk game "Hovertank 3D", in which the player drives a tank through a plane of colored walls and shoots nuclear monsters. In the fall of 1991, after the team—sans Wilbur—had relocated to Madison, Wisconsin, and he had largely finished the engine work for "Commander Keen in Goodbye, Galaxy", Carmack decided to implement a feature from "", a role-playing game in development by Blue Sky Productions. "Ultima Underworld" was planned to display texture-mapped 3D graphics without "Hovertank"s restrictions of flat walls and simple lighting. Deciding that he could add texture mapping without sacrificing the engine's speed or greatly increasing the system requirements as "Underworld" was doing, Carmack enhanced the engine over six weeks from "Hovertank 3D" for another Softdisk game, the November 1991 "Catacomb 3-D". Upon seeing it, Scott Miller of Apogee began to push the team to make a 3D shareware action game.
In November 1991, with the second "Commander Keen" trilogy of episodes nearing completion and their contractual obligations to Softdisk almost finished, id Software sat down to plan out their next major game. Designer Tom Hall, who initially wanted to do a third "Keen" trilogy, recognized that Carmack's programming focus had shifted from the 2D side-scrolling platform game series to 3D action games. After an initial proposal by Hall of a sci-fi project, "It's Green and Pissed", Romero suggested a 3D remake of the 1981 "Castle Wolfenstein". The team was interested in the idea, as Romero, Hall, and John Carmack all had fond memories of the original title and felt the maze-like shooter gameplay fit well with Carmack's 3D game engine, while Adrian Carmack was interested in moving away from the child-friendly art style of "Keen" into something more violent. Encouraged by the reception to his idea, Romero expounded on it by proposing a "loud" and "cool" fast action game where the player could shoot soldiers before dragging and looting their bodies. The core of the gameplay would be fast and simple, for Romero believed that due to the novelty of a 3D game and control scheme, players would not be receptive to more complicated, slow gameplay. He felt the game would occupy a unique place in the industry, which was then dominated by slower simulation and strategy games. Adrian and John Carmack were excited by the prospect, while Hall felt that it was enjoyable enough, and that since he was the company's designer that they could return to his ideas at a later date.