This is one of the 12 pavilions in southern Hokkaido, said to have been built around the 15th century by people of Honshu descent known as Wataritō, as a base for their advance into southern Hokkaido.
At that time, the Watashima Peninsula was under the rule of the Ando clan of Tsugaru, and the area was divided into "Shimonokuni" (centered on Hokuto City (formerly Kamiiso Town)), "Matsumae" (centered on Matsumae Town), and "Kaminokuni" (centered on Kaminokuni Town), each with a governor.
The Shinra no Kiroku, the oldest record of the main road, states that Sueyoshi Kakizaki, the head of the Hanazawa pavilion, firmly defended it during the Battle of Koshamain in 1457.
The main body of the pavilion faces the national highway and is 200 m long from north to south, with a maximum width of 80 m. There is a 50 m difference in elevation from just below to the top of the pavilion. The structure is divided into six large and six small flat sections from south to north.
Around 2,000 coins were collected from the top of the pavilion in 1945, and in 1990, a Suzu ware mortar was collected from the rear of the pavilion. In recent years, partial excavations have been conducted at the rear of the pavilion, including an air-raid shelter, the top, and other flat areas. As a result, a large moat 11m wide and 3.9m deep was found at the rear of the pavilion, and from the top, a ditch, a pillar hole, a 15th century Pearl ware mortar, Chinese celadon, white porcelain, coins, and ironware such as nails and saws were discovered. Ditches and fence ruins were also found in other flat areas. The age of the ceramics indicates that they all date from the 15th century and that the castle had already been abandoned before the Katsuyama-kan site began to function in earnest in the 16th century.
The "Fukuyama Hifu," a record of the Matsumae domain, states that Kakizaki Toshishige died in the summer of 1462, which is almost identical to the document in terms of chronology.
Further full-scale excavation is needed to clarify the full extent of the mansion and its structure.