The Environment GUI actions allow you to change terrain type, but not cliff levels. Furthermore, you might consider using triggers to create walls out of destructibles.
It's a little complicated, but it should be possible to make a trigger that can convert a point on the map into a tile of a specific size, and vice versa, so you can allocate destructibles accurately to create walls.
The whole trigger(s) for making a good, randomized maze is probably very complex. I would recommend checking out unprotected maps that do these sorts of things. For all I know, it's possible to create a random maze, and there are several sollutions.
You could do it with destructibles, for example, or you could do it with flat terrain type only and then make a trigger that kills/does other stuff to units that walk on any other terrain type than grass......... The only thing I don't see possible in GUI is cliffs.