Yes, it is hard to picture alright. The idea of it being a fort is mere speculation, just like most theories to the origin of Yonaguni. It may also be the foundations to a lost city, like something from Atlantis myths. If you want more information on the subject, I recommend you check out Graham Hancock's work on the subject. He did a two part documentary a few years back, but I don't think I have seen it. Still, if you find it, I would love it if you could post it to me Freddy.