The day I left Kumamoto and took the ferry across to Shimabara was a dark rainy day. My raincoat was getting a lot of use on this trip!
Shimabara Castle was built by Matsukura Bungo-no-kami Shigemasa. It took seven years to build and was completed in 1624. The reconstructed five level castle is a local history museum and costs 540 yen to enter. There is an additional fee for parking, but you don't need to drive because it is just a five minute walk from the train station.
At the top level is an observation area. As I said, it was a dark rainy day.
In addition to the five level castle, there are two out buildings, one is a type of art museum and the other has old tools and machines, including an old sewing machine.