What is "chashi"

is the Japanese term for the hilltop fortifications of the Ainu. The word is of Ainu origin, from casi, which means palisade or palisaded compound; a rival theory relates the term to cas. Over 520 chashi have been identified in Hokkaidō, mostly in the eastern regions of the island; others are known from southern Sakhalin and the Kurils; similar phenomena such as the ostrogu of Kamchatka and the gorodische of northeast Asia may have developed independently. A few, including the Tōya casi of present-day Kushiro, date to the Muromachi period; the remainder date largely to the early seventeenth century. As such their construction may be related to increased competition for resources as a result of "intensification of trade" with the Japanese.