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.

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.

Yes, that's what I meant, thanks, and I was thinking particularly of the 2nd game - though on closer inspection I think UW2's walls are all only 45 degree and 90 degree just like the 1st game, except the UW2 map is higher res and a lot better at mixing them up so it's less obvious. So Doom does win there after all, but I didn't notice as a kid. Wolfenstein's are all 90 degree bends, I think.

Both Doom and UW2 were essentially still a 2d map but in UW2 I remember going down a descending corridor and swimming under a bridge I'd previously walked over and being amazed. There's another level full of aerial paths which cross over each other too, and a level where you flick a switch and flood the map and open up some walkways above the flooded area iirc. I don't know how it represented these over-under areas internally but even if no more 2d than Doom (though you could look up and down a bit) it was a nice trick.

Then again, Doom has similar tricks - teleports set up to act like lifts, where the top of the tower looks out apparently over where you've already been (actually a facsimile to get round the 2d addressing).

But yes, Doom did win out for speed and sheer frantic joy/terror. I couldn't convert any friends to Ultima but Doom won everyone who saw it over instantly...