Nostalgia Vector

The Build engine, a sector-based, used by Duke 3D ( and similar to Doom ), had a very bizarre inconsistency...

it was virtually impossible to have two rooms one above another.  It was impossible to see two rooms one above another, not without a great parade of bugs and visual artifacts. That feature was in fact well existing in Duke 3D, and was concealed by having the player unable to see two floors at once. He could either see part of the bottom floor OR part of the top floor, but never both at once in a similar 2D map perspective.

After some reflexion and looking back at things with a new eye, I believe Core Design took care of this problem in Tomb Raider 1 not by making one single map, but by having several.

The Rooms.

The rooms apparently are the trick Core Design figured out on how to operate a sector-based engine in FULL 3D environments, instead of actually stay stuck to 2D.

How did they do it? I do believe they chose to have not just one single map file to operate on, but rather, a cluster of them. See...

This screenshot here is an back-end render ( flawed ) of an area in the Babylon level of my remake. Where we can see pure white colour on the ground, is in fact a separate room undernearth. From a standard sector engine point of view, this looks legit. Why? Because there is no actual 3D to speak of. This "map" section has a definite height, width, length, and inside, we can give it any shape we want.

Second picture is a little more odd, because it's what is underneath this previous room. It does look ugly ( it wasn't meant to look like that ); though, the engine considers this to be another 2D sector map.

The part where Core Design went smart, is when they did this.

By actually cutting a section of the ceilling, we get it to transit with the floor of the room above, in a very similar way we do in the game "Portal". In fact, Lara changes room in a seamless way just like Chell does by crossing a portal in the eponymous game.

This allows to overlap and stack up rooms and sectors in a way the original Build engine from Duke 3D could never do. Core went smart, and yet, we will never see this in modern games.