[wmil]

It was a 2.5D engine, but there were a lot of hacks put in to make it feel more 3D.

For instance you couldn't do proper floors above floors, but there were some rendering tricks you could use to do bridges.

It also supported slanted floors, which helps level designers more than you realize. Also there were invisible teleports to fake a real 3d layout.

But the most important part was that the level designers were very good at using those tricks to hide the limits of the engine.