On the 25th day of my first walk around Kyushu I started out by taking a train and backtracking a ways as I wanted to visit the castle at Obi.
It is not a well-known castle, nor very large, but it is surrounded by a well preserved samurai district and the stone walls are in good condition. The gate house was quite impressive also.
I'm not sure if it ever had a keep but there is a reconstruction of the daimyos "palace" and a museum displaying armour and weapons and such.
For more details on the castles history, access, entry fees etc please see a longer piece I wrote for Japanvisitor.