Quick note on Tilemagic for those that don't really know how to use it yet:
It creates visible tiles as visual effects in the area, it doesn't actually replace tiles. The original tiles/terrain (such as spots with grass) remain in place the entire time.
This also means that Tilemagic effects do not have a walkmesh, so if you raise the Tilemagic terrain, PCs and NPCs will simply walk through it as if it weren't there (they can't stand "on" it, only on the underlying floor/ground).