: How are things like this defined in marathon maps? I mean, if true bridges are impossible, how is room over room like this possible?
Marathon's engine is portal-based. Each line of a polygon is either solid, or a portal connection to another polygon. The renderer chokes if it tries to draw more than one open space along a vertical line, such as the area above and below a bridge, but that's more a rendering limitation rather than a map-format limitation. As long as you can't see vertically overlapping spaces simultaneously, you can actually have them exist.
A fully specified Marathon coordinate is not (x,y) or even (x,y,z), but (x,y,z,polygon). As the player (or monster or projectile) moves across portal boundaries, it updates the polygon coordinate. Within the maps, polygons can overlap not just on top of each other, but fully in (x,y,z) space, which leads to "5-D Space" as seen in the eponymous map, or places like the southeast hall in Couch Fishing. There, you see either the hall or the sunken room, but not both within the same vertical space so it's OK.
