The world geometry is defined in a limited 3D fashion (but still 3D since while the walls are defined in a 2D plane, there is also sector floor and ceiling heig

The world geometry is defined in a limited 3D fashion (but still 3D since while the walls are defined in a 2D plane, there is also sector floor and ceiling heights which add the third dimension - a modern equivalent would be terrain that is almost always a heightmap that can't create caves etc - these are done with additional geometry and clever hiding of the seams by the artists) but the objects are placed in three dimensions. However some calculations only take into account the two dimensions, e.g. object-to-object collision is often done on the XY plane, hence you can be "blocked" by enemies or pillars or other objects that are "below" or "above" you.