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.
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.