a concept that's fun to think about is an "unplace" (a term i am making up).
in most "worlds", things exist "off the map", so to speak. there might not be anything there, but it exists. if you go off the edge, you might end up nowhere, or an empty room, sure. but you can go there.
like in real life, you can technically go anywhere. everywhere exists; even if it's solid rock or empty sky, you can, in theory, move to that position and exist there.
in this world, that isn't the case. empty spots on this map do not exist, in any shape or form. you cannot go there. under no circumstances can you "boundary break" and visit one of the empty spots on this map.
(of course, this whole map is a fabrication; it does not exist in the game world.)
consider link's awakening, above; this is eagle's tower, the seventh dungeon, from an "internal" perspective (src). the map is stored as an 8×8-byte array of room IDs. the repeated room you see everywhere is room 00; on the first underworld, it's an empty room, but in the second underworld, it ended up being a copy of the top-left corner of 1F.
what you see here is actually how this game works. if you are on one screen, and walk off of it, whether by cheating or otherwise, you will move... one screen to the right. the "empty" screens are still there, even if they're not intended to be visited. what you see is what you get.
in the case of mighty bomb jack... that isn't how it works. the "maps" people, including myself, have made, aren't actually representative of the world. much like how a 2d projection of the globe has to take some affordances to account for the loss of its 3dness, mighty bomb jack's map is a transformation of how it works into a "normal" representation.
take the labyrinth on the right. the middle, leftmost room, D1, is adjacent to D8 (right). but you cannot get to D8 from D1. i don't just mean that there's a wall there: i mean even if you were to insert a door into that wall, and go through it, you would end up in a void1, room 00. and from there, you're literally off the map -- room 00 isn't "on here", because it isn't anywhere.
to put it another way, when you're standing in D8, the only other rooms that exist for you are DE and D9. "the world" doesn't exist: the world, quite literally, revolves around you.
it's just weird to think about, when the game world otherwise fits together 1:1. it feels like it should work like any other map. it just doesn't. and it's fun to pontificate about the deeper meanings of that.
(1. you would have to alter the room-neighbors table to make it work, which is kind of the crux of this issue: it's not a coordinate plane.)
this distinction exists all over the place too. duke nukem 3d, from my understanding, is the first game that Thought With Portals - the map is a manifold of connected sectors, and while the map editor and the map viewer render them in a single coordinate space, the game engine parses them into something very different. each sector is its own independent universe, and sector boundaries represent connections between them; there is no simple euclidean relationship between regions.
you can see this in the very first level, when you go up a spiral staircase into the projection room. the second loop of the staircase is above the first; the game engine does not support this. the projection room is above the foyer; the game engine does not support this. the renderer doesn't comprehend two spaces occupying the same X/Y while differing only on Z - yet it works. and in fact the Z difference doesn't really matter either - you can easily make a map with many rooms occupying the exact same plane.
this works because as you walk up the staircase or traverse through the hallways, the areas you were just in cease to exist, and the ones you're approaching materialize. except... that's not really true. it's a convenient way to think about it, but it's wrong; those areas aren't removed from memory, there is no explicit alteration of the map occurring, it's just that the shape of the world is incomprehensible to our 3D minds. people really want it to Make Sense, but it won't as long as you try to make it work like reality.
people on various forums (none of which have studied the game's code) argue that part of why the spiral staircase trick works is because the height of the sectors prevents them from overlapping, but I don't think that's true. You could make the player go up a dozen flights of extremely short stairs that only lift them a few inches on each turn, and still never see any overlap, because it simply isn't a euclidean space. this isn't just a feature of the renderer, but a pervasive quality of the engine.
to wit, in the portal test video linked above, when the player passes from one room to another and then crosses it, they're walking right through spaces that contain hard "impassable" walls, if you were to look at them in the build editor. those walls aren't checked by the collision code however, because they aren't in the sector the player is in.
i can't stress enough that this is not "a trick", as some people like to describe it. it's not just that "the editor doesn't have a way to display 3d," it's that it isn't creating 3D. there is no way to "unfold" that demo map into a "real" space; any attempt would be distorted and misleading. and for that reason, the spiral staircase and projection room are no more "real" - the change in height allows us to accept that they make sense, but the map designers could have simply put the projection room at the same level as the foyer and it all would have fit together just fine. though I'm not sure the window to the theater could have worked - more research is required.
in any case: doom was not like this, it worked more like Link's Awakening. the map is a strict coordinate grid, the player occupies a point on that grid, and at any given point there is one authoritative sector. all objects in that space are snapped to its floor height, and that's the end of it. I have seen this handwaved as "well doom uses BSP so it has to work that way," and I don't have the basis in data structures to say that's bullshit, but I'm pretty sure it's an oversimplification. BSP went on to be used in many other engines, many of which can happily support non euclidean spaces - like Doom 3, which was used for Prey, or Source, which was used for, you know, Portal.
Footnote: yes, I mean Z, not Y; the source code in these games uses a top-down coordinate system.
