The gameplay of "Heretic" is heavily derived from "Doom", with a level-based structure and an emphasis on finding the proper keys to progress. Many weapons are similar to those from "Doom"; the early weapons in particular are near-exact copies in functionality to those seen in "Doom". Raven added a number of features to "Heretic" that differentiated it from "Doom", notably interactive environments, such as rushing water that pushes the player along, and inventory items. In "Heretic", the player can pick up many different items to use at their discretion. These items range from health potions to the "morph ovum", which transforms enemies into chickens. One of the most notable pickups that can be found is the "Tome of Power" which 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.