Hi all, just saw this on youtube and actually found it quite interesting, especially as how I loved ID Software’s shooters back in the day! Moreover, I think the solutions they came up with were ingenious, and who knows, may even inspire some ideas 
Some call Doom 2.5D. The maps were 2D, and so were the sprites (rendered in billboard mode so they always faced the viewer). The map allowed for ceiling height and floor height, but in the first Doom, no slopes, and of course, no “bridges” (where you could walk over it and walk under it, so an X-Y coordinate had two floors and two ceilings) since that would require a genuine 3D structure. Duke Nukem 3D added some hacks so limited bridges and slopes were possible, but sprites were still 2D (though some had a different view for each direction to simulate 3D views). I think quake was the first genuine 3D.
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