I've picked up a couple books from the Japan Foundation Library (here in Toronto) that look interesting. The first book is 'Castles of the Samurai: Power and Beauty' by Jennifer Mitchelhill (Kodansha International Ltd, 2003). It has lots of lovely colour plates of various castles. Another interesting aspect of the book is in its appendices. It lists the mon (clan symbol), castle name, its Province and its income (in koku) for the Edo Period (1848 ). That info could be useful for a Japanese based RPG game.

The second book is called 'Japanese Castles' by Motoo Hinago, translated by William H. Coaldrake (Kodansha International Ltd, 1986). it has lots of useful history and even floorplans and side views of various castles. The colour plates are great to look at as well.

I think both of these books could be a useful resource for anyone interested in Japanese Castles. I also got a nice, big book called 'Minka: Traditional Houses of Rural Japan' by Chuji Kawashima, translated by Lynne E. Riggs (Kodansha International Ltd, 1986). It has many floorplans of traditional Japanese homes. I can see it inspiring a map or three from me.