[mthoms]

Neither of those were true 3D. In fact, even Doom wasn't fully 3D.

The view could only ever be at 90 degrees for example (no looking up or down) and the environments couldn't have more than one floor and one ceiling at any given location.

I think the limitations made the genre even more successful because it forced people to "evolve" into 3D by going from Wolf3D > Doom > Quake. Step by step. True immersive 3D was actually a jarring (but undoubtedly fun) experience for the first time. Anyone here remember Descent?

[int_19h]

By Heretic, you could look up or down in Doom engine, although the way that was done distorted perspective (it basically kept all vertical lines vertical, while distorting proportions horizontally, to approximate the effect).

I would also add Duke Nukem 3D to your step-by-step list. Reason being, firstly, it had sloped floors. But also, as I recall, it was the first 3D action game to have what was then called "room over room", even though the engine was fundamentally limited in the same manner as Doom - it achieved that by utilizing portals. So actual architecture was still one-level, but portals transparently connected it in a way that simulated multiple levels.

There were also some really crazy hacks to achieve something like that in Doom. Like, bridges that you could walk over or under. The way it was done is by having an invisible platform that was lowered and raised as the player approached the "bridge" from the corresponding direction, combined with a fake middle texture that wasn't actually solid.

[mthoms]

All this talk of Id games and I totally forgot about Duke! It definitely should be in the discussion. Now, if they'd only make a sequel... ;)

[james_s_tayler]

Descent was another favorite of mine! Really used to get my heart racing at the end of every level when you were close but you had to brave the final room and manage to escape before the countdown.