1) This table was compiled out of images from Google Maps and information from two sources. The first of those was my own visits to these fortresses in March of 2013. Unless otherwise noted, the dates for these fortresses were obtained from inscriptions over the doors of the fortresses. The other source is a book called "Finding Fortresses in Yu Xian"/找寻蔚县古堡, written by Lin Shengli 林胜利. To be perfectly honest I don't trust the dates in 'Finding Fortresses'; the book sites no sources, and it's not clear how he obtained dates for some of the fortresses which have no inscriptions. Therefore take some of these with a grain of salt. The point of making this chart to begin with was to establish a typology that would help understand the changes in fortress morphology over time; hopefully if I had that (or just went and found this guy) I could second-guess his numbers. Which leads me to:

2) The accepted history of fortification in Yu Xian goes something like this. Yu Xian is in an interesting geographical position, because it is set somewhat back from the Sino-Mongol frontier but not cut off from it by any high mountains. Thus it was not subject to the kind of massive, centralized military fortification projects that took place along the Great Wall lines in the valleys north of it, but nor could it afford to be unprepared for attack. There were certainly fortresses in Yu Xian before the Ming dynasty (1368-1644) but it seems hard to say how many there were or what they looked like; I'm frankly skeptical of the ones that 'Finding Fortresses' claims are pre-Ming. The Ming fortresses were built during two main periods. (1) During the Hongwu reign (1368-1398) at the start of the Ming, and in the chaotic hundred years between the Chenghua and the Jiajing reigns (1464-1567), peaking in the Jiajing.

3) It's my contention that each of these periods can be associated with a certain type of fortress, and that this tells you something interesting about village organization and militarization. Whether or not this chronology actually proves anything at all, you can be the judge. I think it sort of does. The Hongwu and supposed pre-Ming village fortresses tended to be bigger, and to have irregular walls and street plans. There were also a lot of small hill forts built during the Hongwu reign, which were obviously used by the military to defend the access routes to the basin of Yu Xian. At the start of the Chenghua reign, the villages suddenly become much more standardized, smaller, with extremely rigid designs for walls and streets. This was presumably due to the influence of people like Yin Geng 尹耕, although since he was only born in 1515 he can't have been originally responsible for what was obviously a bit of a sea change in fortress building. This whole picture is kind of muddied by the lack of reliable sources for anything pre Chenghua (I never saw any inscription that predated that reign) and now that I write it down I wonder if it was worth all that trouble just to somewhat demonstrate this. Anyway.