The castle was built in 1591 by order of General Toyotomi Hideyoshi to serve as a basecamp for the (failed) invasion of Korea and China; it was later abandoned and dismantled. All that remains of the huge original building are the foundations and outer walls. The nearby museum has a digital tablet with a virtual view of how the fortress used to look as you walk around. The main plateau commands a stunning view of the Genkai Sea.