Rooms are the basic 3D models that compose a stage. Each stage is constructed of one or more rooms. Each room, in turn, is made up of a list of vertex data (vertices) and up to two display lists. Rooms are rectangular in shape and positioned seperately within the stage, relative to the centerpoint 0,0,0. Each room is independant of every other room. In order to see between each room, either a portal is set between the rooms or special visibility is used. Rooms themselves have no boundries and do not interact with objects or players. Instead, clipping is used to lay out a playable area, designate which rooms appear when the player is in a certain location, and set the lighting within the room.
Each room is made of either one or two display lists, and each display list has its own function. The first, often called a primary display list, builds the basic structure of the room. Gunfire and objects interact with any geometry built using the first kind of display list. Secondary display lists are not required but have special features that make them useful. Gunfire and objects do not usually interact with this kind of geometry. Often, transparent objects will be constructed in this manner, such as railings, grating, or antennae. In addition, glass and lighting can be built in such a way that it reacts to gunfire the same way objects do. This is seen in the hallways of Bunker and laboratories in Silo, for instance.