"Chashi" means "fenced enclosure" in the Ainu language, and it is believed that they were used for multiple purposes such as a fort, a place of ritual, and a lookout post.

Approximately 500 castle ruins have been confirmed in Hokkaido, with 32 remaining in Nemuro city, 24 of which are designated as national historic sites as part of the "Nemuro Peninsula Chashi Ruins Group." In 2007, it was also selected as one of Japan's 100 Greatest Castles (castle number 1) by the Japan Castle Foundation. The exact date when the castle ruins in Nemuro city were built is unknown, but they are believed to be from around the 16th to 18th centuries.

Nemuro city is known for the large number of "cliff-like" castle ruins with semicircular or square moats surrounding them on cliffs overlooking the sea, and for the large number of large-scale ruins with multiple moats. Currently, there are two ruins that have been developed as sightseeing spots: Notsukamafu No. 1 and No. 2 Chashi ruins and Onnemoto Chashi ruins.