If you’re interested in Japanese Castles, you should definitely check out this site: Jcastle.info.
I love this site because it’s organized really well. For example, if I’m travelling in Japan and I want to know if there is (or was) a castle in the area, I can just search by area.
Since I live in Tokyo (and Japan This! is primarily concerned with Tokyo), there is a special grouping on the front page for Tokyo Area castles.
If you want to visit Japanese Castles and you’re not sure which ones are worth the trip you can check out his 5 star castles and 4 star castles.
There aren’t a lot of people blogging and running websites about Japanese History, so if you are a fan of Japanese History, you should subscribe and support as many quality sites as you can. This one is definitely worth your time.
Oh, I almost forgot! Jcastle is also very newbie friendly. There are a lot of specialized terms that have to be used when discussing castles. He’s got dedicated section to names and types of structures that you’ll see when you visit Japanese castles. For example, wtf is a 天守閣 tenshukaku or a 唐破風 karahafū. (I’m not going to tell you, you’ll have to click the links and visit the site for yourself to find out).