More maprestriction in next AC?

[V-Man]

You have to remember that Cube 1 maps aren't truly 3D.

Someone explain this notion to me. Is this referring to the way lighting is done, that each cube's(And entity's) position on the X&Y axes is locked, both, or something else?

From what I can see in the map engine, that's basically it.

Here's how I understand it. It may not be totally correct (though I couldn't tell you where), but it has made my brain hurt less to think about it in these terms.
The information about each map (.cgz file) is basically 2-dimensional, with cubes and entities having a mysterious property referred to as "height." Treating height like an exotic property allows the cgz files to save dramatic amounts of room, keeping them nice and compact, even for very large maps.
Once the map is plugged into the 3d-rendering engine of AssaultCube, however, the "height" property is interpreted as an additional z-axis for the viewer's eyes and voila! A 2-dimensional map is given a 3-dimensional interpretation. The price of interpreting height in this manner can be seen as an increase in graphics statistics -- client-side lag -- if too many entities and cubes display height. This is why having more solids and fewer mapmodels can eliminate lag: solids don't display height properties.
Hence, Cube 1 maps are basically 2-dimensional. Care should be taken to avoid saying that AssaultCube itself is 2-dimensional, however -- or else the players wouldn't be able to jump up onto the platforms.