:^) Ya build with what ya have. Post-Roman construction in Britain was wattle-and-daub walls and thatch roofs, that may have been in introduction by the Anglo-Saxons, but was likely commonplace for a really, really long time, and not just in the British Isles. When your house gets burned down a few times, and/or you have some cold winters, stone looks like a better idea. Also, the remains of mud huts tend to be a bit ephemeral -- more of them at the time, but the remains are more fragile as well as easily missed.
I understand that point, the geometry is really what I was wondering about, rectilineal vice round.