Gassan Hiwa Castle is said to have been built by the commander Kimotsuki Kaneshige during the Nanboku-cho period, and Hongo Tadasuke, the 8th lord of Hongo clan who settled the Miyakonojo basin, spent his last years. Its apperance imitates the Inuyama Castle with three-tiered tower structure, where you can see not only Miyakonojo Basin but also Kirishima Mountains and Sakurajima from the observatory.
The museum built on the site displays historical and folk material from Jomon period to the modern times of the former Takajo town.