Thanks Maxboy.

Edit>Transform>Warp. You don't say if that's Gimp you're using, but if not I'm sure Gimp will do something similar. I'll have a play with that, first chance I get.

Obviously they can't be scaled, cos the size of the stones would be wrong, so it'll mean drawing them out individually at different radii, but I only have to make one quarter circle at each radius, X and Y flips will take care of the rest. A bit of pi D / 4 and hey presto, circular towers.