Here’s something about the history of Kumamoto. From around noon on July 1 of 1945 into the following night 154 B29 bombers dropped 1107.4 tons of bombs on Kumamoto City. According to this page, 469 died, 552, were injured, 11,000 homes were damaged or destroyed, directly affecting about 43,000 people. The pink areas you see on the map were burned. If you go to the page linked to here you can see an aerial image of the result of the bombing. Kumamoto was also bombed on August 10th, the day after the nuclear bombing of Nagasaki.
Looking at the map of the burned areas can help you understand why so few parts of Kumamoto have buildings that were put up before 1945.
I recently read an essay by HARADA Masazumi describing his experience in the bombing, which killed his mother.
http://www.city.kumamoto.kumamoto.jp/content/web/asp/kiji_detail.asp?ID=2662&mid=1&LS=132
