The gameplay of
Heretic is heavily derived from
Doom, with a level-based structure, an armor system, triggers which open concealed chambers filled with enemies, and an emphasis on finding the proper keys to progress. Raven added a number of features to
Heretic that differentiated it from
Doom, notably interactive environments, such as rushing water that pushes the player character along, and inventory items. In
Heretic, the player can pick up certain items to use at their discretion. These items range from health potions to the "morph ovum", which transforms enemies into chickens. The "Tome of Power" acts as a secondary firing mode for certain weapons, resulting in a much more powerful projectile from each weapon, some of which change the look of the projectile entirely.
Heretic also features an improved version of the
Doom engine, sporting the ability to look up and down within constraints, as well as fly. However, the rendering method for looking up and down merely uses a proportional pixel-shearing effect rather than any new rendering algorithm, which distorts the view considerably when looking at high-elevation angles. As with
Doom,
Heretic contains various cheat codes that allow the player to be invulnerable, obtain every weapon, be able to instantly kill every monster in a particular level, and several other abilities. If the player uses the "all weapons and keys" cheat ("IDKFA") from
Doom, a message appears warning the player against cheating and takes away all of their weapons, leaving them with only a quarterstaff. If the player uses the "god mode" cheat ("IDDQD") from
Doom, the game will display a message saying "Trying to cheat, eh? Now you die!" and kills the player character. The original shareware release of
Heretic came bundled with support for
online multiplayer through the then new
DWANGO service. ==Development==