Freedoom is a free first-person shooter under continuous development. It is based on the same structure as the Doom II IWAD.
It was possible because id Software released the source code of the Doom engine in 1997 under the GPL license. This allows any custom levels or other customizations designed for Doom II (e.g. PWAD files) to be used with Freedoom. Other variants are available that have the same structure as the original Doom and its shareware version.
As downloaded from the official Freedoom site, it is distributed as an IWAD (game data) file only without an engine. The player must provide a Doom source port such as PrBoom, ZDoom, Boom, or any other Boom-compatible source port. Freedoom requires an engine with Boom extensions and will not work with the original Doom engine's source release.
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