In late May, the Asahi Shimbun (Japanese version) published an article about what happened with the “ippon ishigaki” (one-legged stone wall) that supported Iidamaru Gokai Yagura. It seems it had been rebuilt after an earthquake that occurred in 1877 and that the workmanship was inferior to the original. For example, instead of fitting irregularly shaped stones together, rectangular stones had been used. Moreover, small gaps between the rocks further weakened the structure. The article describes other factors that seem to have been behind the collapse but doesn’t say why the corner held out. I would guess that the corner was put together in a more traditional way but I’m not sure.
– Kirk