I spent last week working on a couple rooms of a long infantry barracks. 4th century on the top, 3rd on the lower layers. This week, something different. Sitting on top of the north end of the barracks is an odd, jumbled pile of rough looking stones. It's.....something.
It is more or less round. The stuff off to the left is plain old dirt. On the right it is set on top of the barracks structures, which like everything the Romans built, were made with straight lines and right angles. Well, lets say almost everything the Romans built was like that.
Here's one exception. Vindolanda has a large series of these round structures. The purpose is a mystery but they were built in the time of the Severan campaigns and may have held British loyalists to the Imperial cause. Or prisoners. British folks lived in round houses after all.
Ah, but this critter is several layers higher, and probably a couple of centuries later, so not likely. Well, how about.....
Late Roman and post Roman churches had rounded sides. This is one from the other end of the site. Certainly a candidate. Not too many other possibilities exist. Sometimes Roman work shops would have a rounded apse. Probably where the forge or furnace was. It's a odd thing to look for in a probable Post Roman context, but as it happens the technology involved in making iron into nails and weapons survived into the Dark Ages.
So.....I'll update when I can, but at the end of the day's work something interesting did turn up to suggest an answer..........