"Chashi" means "fence enclosure" in Ainu language, and it is believed to have been used as a fortress, a place for rituals, a watchtower, and for many other purposes.

About 500 chashi sites have been confirmed in Hokkaido, and 32 of them remain in Nemuro City, 24 of which are designated as "Nemuro Peninsula Chashi Site Group" as a National Historic Site. In 2007, the Nemuro Peninsula was selected as one of the 100 best castles in Japan (castle number 1) by the Japan Castle Association. The exact date of construction of the chashi ruins in Nemuro is unknown, but it is believed to have been between the 16th and 18th centuries.

Many of the chashi sites in Nemuro City are known for their large scale moats, which were built on cliffs overlooking the sea and surrounded by semi-circular or square moats, often in combination with moats. Currently, there are two sites that are open for tours: the Notsukamahu No. 1 and No. 2 chashi sites and the Wonnemoto chashi site.