> maybe this limitations turns into the explanation that its 2d?

But it still not 2D. These types of engines are referred as 2.5D since ancient times specifically because they aren't full 3D but they aren't 2D either.

> You can't look up and down either, so your shots go twords the enemies hieght.

This is a limitation of implementation. Duke3D, built on the exactly same principles, allows you look around, and ROTT too.

> but I remeber some discussion of a restrictions where can't have a room directly above another room

> where you can can go over and under (like a bridge).

You can't have a proper 3D bridge, but you can place a sprite and walk over it, this technique is used by a later DOOM engine mods, Duke3D, ROTT [0]

RoR isn't that difficult to implement in a 2.5D engine, you just need to keep track in which one the entities are located (and Z axis helps here a lot).

Overall, all that 2D/2.5D/3D bickering is pointless if you understand what while DOOM, Duke3D, ROTT engines are 3D, their map layout (specifically the map geometry, not the entities) is 2D only, though with an additional 'ceiling height' and 'floor height' property per sector hence 2.5D.

[0] https://youtu.be/kfKwItTc44I?t=955

The world geometry is defined in a limited 3D fashion (but still 3D since while the walls are defined in a 2D plane, there is also sector floor and ceiling heights which add the third dimension - a modern equivalent would be terrain that is almost always a heightmap that can't create caves etc - these are done with additional geometry and clever hiding of the seams by the artists) but the objects are placed in three dimensions.

However some calculations only take into account the two dimensions, e.g. object-to-object collision is often done on the XY plane, hence you can be "blocked" by enemies or pillars or other objects that are "below" or "above" you.

All depends on what part of the game you consider:

Level geometry is indeed 2D (or rather 2D projectable without intersection, hence the 2.5D moniker)

Gameplay though, is 3D, and actually hampered by not being able to look up down, which for shooting was compensated by vertical auto-aim. There are many places where it's hard to see the enemies above or below, or even where you're going to go/navigate. Luckily the level design is usually nicely done and usually caters for that with enough room space to kinda see things and/or elevators but there are a couple of areas that can be especially tricky or frustrating.