Page "Doom (video game)" Paragraph 25

Another important feature of the Doom engine is its modular data files, which allow most of the game's content to be replaced by loading custom WAD files.

Wolfenstein 3D was not designed to be expandable, but fans had nevertheless figured out how to create their own levels for it, and Doom was designed to further extend the possibilities.

The ability to create custom scenarios contributed significantly to the game's popularity ( see the section on WADs, below ).