Oh it is possible. I'll have to look through my old files to see how I did it before, but I was able to edit terrain, and place buildings. It has been years though... Placing building in skirmish maps is considerable easier though. [[edit]] Scratch that, turns out I did still have notes from the time. Best I could come up with was editing terrain. My attempts at changing buildings only seemed to corrupt the maps. I was however able to place them in skirmish. Walls, Turrets and such don't tend to bug anything, but some other buildings do.
