The Freedoom project aims to create a complete free content first person
shooter game. But Freedoom by itself is just the raw material for a
game: it must be paired with a compatible game engine to be played.
id Software released the source code to their classic game, Doom, under
the GNU General Public License. This means that the program code that
powers Doom is free; Freedoom complements this with free levels,
artwork, sound effects and music to make a completely free game.
Phase 1 is the first part of the single-player game, containing four
chapters which are nine levels each, smoothly paced for beginner
players. It is compatible with mods for the original Doom and The
Phase 2 is the second major part of the single-player game, a massive
32-level chapter expanding upon the same concepts present in Phase 1, as
well as containing additional monsters and the double-barreled shotgun!
The levels in this game tend to demand more skillful play than the
previous chapters. It is compatible with mods for Doom II.