Chashi
is the Japanese term for the hilltop fortifications of the Ainu. The word is of Ainu origin, from チャシ (casi, ), which means palisade or palisaded compound; a rival theory relates this to the Korean term 잣 (cas, jat, ) of roughly the same meaning. 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. Form The early Dutch explorer Maarten Gerritsz Vries described the chashi he encountered in eastern Hokkaidō in 1643:These forts were made as follows: on the mountain on which they were placed was a small road steep to climb, and round on the four sides palisades were placed of the height … of 1½ man's length; within this stood two or three houses. There were large fir doors