Marathon 2: Durandal is the sequel to the first Marathon. Although the engine is similar to Doom in most respects (2.5D maps, bitmap sprites instead of polygons, etc.), Marathon 2 distinguishes itself with such features such as a rudimentary inventory system, ambient sound, multiple fire modes for each weapon, allies who aid the player, enemies belonging to different factions which will attack each other, a more developed plot (gradually narrated to the player at various computer terminals scattered throughout the levels). 
