Stephen Turnbull's Japanese Castles in Korea is definitely one of these weird unknown subject matter that is worthy of Osprey's Fortress Series. In this short book, Turnbull managed to give a pretty good summary account of history of Japanese castles that were built during Hideyoshi's Korean invasion between the years 1592 to 1598. The book explained how these castles were built initially to support the invasion, support the supply lines, to control and policed the area around it and finally to support the withdrawal of Japanese troops from Korea after Hideyoshi's death. Stephen Turnbull managed to do all this in this short book with clarity and understanding that don't bogged the reader down. Turnbull also stated that Japanese castle designs at that time proves to be quite capable of withstanding the might of the Ming armies from China. From what I understand, lack of artillery consideration appears to be the major weakness of the Japanese military when defending their castles. Still, three major sieges of Japanese castles in Korea all ended with Japanese victories. And according to the author, the Japanese forces also adapted Korean cannons to their defensive lines as well.