i think that there is now a way that you can compile the level with concave blocks.. but this may be only in A7 .. the problem you are having is that the BSP engine does not allow blocks to have inndents in them. like a cave is an indent in a cliff.. you could try using models .. or scaling the block larger thatn you need and then moving the points back to where youu want them.. or only use the face and edge manip tools....