Is it possible to stop flying units flying higher when going over cliff?

I want to make it so a flying unit will not go over mountains, he will go through them. Is this possible?

region the whole mountain and when it enters region take away its altitude somehow?

The guys at the fps mod created an intesresting little program, it can find the actual height of any unit. Using that program you can get units to fly at the same height over mountains.

I'm not sure that's possible; all units respond to the terrain, flying or not. Could you use doodads? Flying units do NOT respond to doodads, and will clip right through them - if you sized up a bunch of rock chunks and placed them creatively, you could make a mountain pretty easily.