compared to the other early-20s employees and his religious background. The team also added a third programmer, Dave Taylor. Petersen and Romero designed the rest of the levels for Doom, with different aims: the team felt that Petersen's designs were more technically interesting and varied, while Romero's were more aesthetically interesting. Romero's level design process was to build a level or part of a level, starting at the beginning, then play through it and iterate on the design, so that by the time he was satisfied with the flow and playability of the level he had played it "a thousand times". The first level, made by Romero, was the last created, intended to show off the new elements of the engine. The ending screen of each level, like in Wolfenstein 3D, displays a "par time" for the level, as set by Romero. In late 1993, after the multiplayer component was coded, the development team began playing four-player multiplayer games matches, which Romero termed "deathmatch"; he proposed adding a cooperative multiplayer mode as well. According to Romero, the deathmatch mode was inspired by fighting games. The team frequently played Street Fighter II, Fatal Fury and Art of Fighting during breaks, while developing elaborate rules involving trash-talk and smashing furniture or equipment. Romero later stated that "you could say that Japanese fighting games fueled the creative impulse to create deathmatch in our shooters". Programming Doom was programmed largely in the ANSI C language, with a few elements in assembly language, on NeXT computers running the NeXTSTEP operating system. The data, including level designs and graphics files, is stored in WAD files, short for "Where's All the Data". This allows for any part of the design to be changed without needing to adjust the engine code. Carmack had been impressed by the modifications made by fans of Wolfenstein 3D, and wanted to support that with an easily swappable file structure, and released the map editor online. Romero and Carmack spent the early stage of development focusing on engine features instead of the game concept. Wolfenstein had required levels to be a flat plane, with walls at the same height and at right angles; while the Doom world was still a variation on a flat plane, in that two traversable areas could not be on top of each other, it could have walls and floors at any angle or height, allowing greater level design