[G3mInI]

if any of the brushes you are csg'ng touch terrain anywhere MOHradiant will crash. Now with that said you do have options...

first off i wouldnt use csg to make an 'opening'. Try using the clipper tool. May be intimidating at first but is quite useful.

secondly if you insist on using csg... then highlight all your terrain and save it to a prefab. Once saved highlight it again and delete it from the map. Dont worry you have it saved as a prefab. Now that the terrain is gone go ahead and do all the csg you like. When completed just load the prefab of your terrain and it will come back in the same place it was when you saved it.

good luck

-g3mini