Hmm I had time for only a brief play, but Gimp doesn't seem to have an equivalent function (not that my noob skills can find, anyway) but I got a similar effect using Filters>Distort>Polar coordinates.

Unfortunately it seems to distort the stones into what amounts to a string of sausages, and I didn't have time to blow the thing up to see if it was a problem on a higher zoom. The ratio of wall length to image length determines how much of a full circle it makes.

I wonder if I'd have any better luck with Inkscape?

Of course, if Torstan's gonna play with circular walls, I could always sit and wait...