Mapping southern Gansu Province, China, I’ve encountered dozens of closed walls, in shapes usually square or oval. In some areas, every village has it’s own ‘castle’. So far, the only thing I’ve read about them was this:

“After the fall of the Qing-dynasty, when the country was undergoing an epoch of chaos, the farmers in the area were being constantly terrorized by local warlords and bandit hordes. So they hurriedly erected these fortifications on the highest points in the hills, thus having a safe haven to fall back to in case of an attack.” Source (with a few pictures)

Here is another picture up close. It even seems to be made from rammed earth, which could mean that some of them may be older than they may seem at first sight.

Edit: A kind OSM contributor sent me a link to a documentary about these structures. It’s in Chinese, but with English subtitles.

http://tv.cctv.com/2016/03/22/VIDE2oprFGUUcK44tKYNHfGX160322.shtml

I haven’t had the time to watch the entire documentary, but I got as far as that parts of these structures may be many centuries old.

I wonder if some of them may have already been constructed during the muslim rebellions a few decades earlier.

So far I’ve tagged hem as ‘historic=fort’, ‘barrier=wall’. Use this Overpass query to find them. A few that had noticeable decay have been tagged as ‘historic=ruins’.

If you have more information on these structures I’d be happy to hear so (may also be in Chinese).