The Nemuro Peninsula has the densest surviving group of Ainu chashi in Hokkaido. These earthwork sites are often described as forts, though they may also have had ritual or lookout roles, and many sit near cliffs with moats and low banks rather than stone walls or reconstructed buildings.
Do not expect a castle in the usual Japanese sense. The appeal is quieter: grassy outlines, sea wind, and the knowledge that the peninsula was already a contested and organised landscape long before modern borders arrived. Onnemoto Chashi is one of the easier sites to understand on the ground.