Your Fortresses of clay remind me of similar structures my friends and I used to make on tiny aights and sandbars in the old creeks (in our tongue, "Cricks") we frequented strange aeons ago. We crickrats used the big, smooth round cobbles for the walls, and we used slate and phyllite (we used to call it "gypsum" but it is crumbly and very sparkly, it ought to be called "elfenstone" or "elfenshale") for the roof shingles. These were no more than 3 feet high, and most were hovels no bigger than 2 feet in circumference. We plastered them together with Indian clay / Gault clay from the crick beds. They never stayed for too long because we made them during the middle of summer (mid-late July) and that was essentially thunderstorm season in the Southeastern Keystone State. We considered it a minor victory if any part of our "forts" withstood the summer rains that repeatedly flooded the creek. Furthermore, whoever built the sturdiest fort usually incurred the wrath of the others,
who would attack it like angry cyclopes hurling large wet rocks at it until it crumbled under the seige with one last gritty, sedimentary sigh. I have no photos of these stoney, clayey domes, though I do have many happy memories sloshing about in the crick while making them on those hot, steamy summer days.
I truly believe that these structures form some megalithic reflex that crops up in one's conculture repertoire!