Vertigo Secret Level 3, is there any way to access the highlighted area?

It would appear that this is one of those odd busted ones I mentioned in your other question!

The cube in question is highlighted. The Red/Yellow square is actually bugged out such that there is no texture present. Adding a new cube in the editor fixes this, but the DLE-XP editor is a much more robust editor than the original D1 and D2 editor, so likely this was missed.

The "wall" attached to that cube, indicated by the grey cross pattern, is the locked door you encounter in mission. Editor-wise, it is actually a door of type "Close", which means it basically never opens. There is no trigger attached to this door either. There is also no objects inside of that cube (powerups, robots, keys, etc). It's just a dud!