: The file format is
: completely 2D with floor and ceiling elevations applied to completely 2D
: polygons
: So no, Marathon is far more 2D than it is 3D; it's just got a lot more neat
: tricks that make it SEEM more 3D than PiD.
Marathon isn't truly 3D, but it's definitely more 3D than Pathways. Elements of Marathon do have 3 coordinates and take all of them into account- a monster can fly over your head without colliding with you (unlike in Doom). Some projectile options do modify all 3 dimensions at once, like wandering or guiding. Map vertices have 3 coordinates that make them unique, you can have vertices that differ only in Z.
This is exactly what "2.5D" refers to- games that involve all 3 dimensions but treat one of them differently from the other 2. That's not the same as completely ignoring it like in first-generation 3D games (Pathways and Wolfenstein). If it was possible to do everything in Z that could be done in X and Y, then Marathon would be a third-generation true 3D game (but no one's claiming it is).