Doom (1993 video game)
The ability for others to create custom levels and otherwise modify the game using WAD files turned out to be a popular aspect of "Doom". Gaining the first large mod-making community, "Doom" affected the culture surrounding first-person shooters, and also the industry. Several future professional game designers started their careers making "Doom" WADs as a hobby, among them Tim Willits, who later became the lead designer at id Software.
Doom (1993 video game)
"Doom" was programmed largely in the ANSI C programming language, with a few elements done in assembly language. Development was done on NeXT computers running the NeXTSTEP operating system. The data used by the game engine, including both level designs and graphics files, are all stored in WAD files, short for "Where's All the Data". This allows for any part of the game's design to be easily changed without needing to adjust the engine code. Carmack designed this system specifically to enable fans to be able to easily modify the game; he had been impressed by the modifications made by fans of "Wolfenstein 3D", and wanted to support that with an easily swappable file structure along with releasing the map editor online.