Ryukyu and Okinawa Environment. A History. The Ancient Ryukyus generally refers to the period from the 12th century, at the beginning of the Gusuku (Castle) Period, through to the establishment of the Ryukyu Kingdom, to the invasion of the Ryukyus by the Shimazu Clan from Kyushu in 1609. In 12th century Okinawa, the long Shell Mound period, where people lived a hunter-gatherer lifestyle, was coming to an end as an agrarian society was formed. In conjunction with this, regional chieftains, called Aji, gathered political power in their respective areas, built castles, called Gusuku, for fortresses and struggles for overall political power began. This is called the Gusuku Period. By the 14th century three great houses had developed and Okinawa was divided into three centers of power; the south, called Nanzan, the central area, called Chuzan, and the north, called Hokuzan. This was called the Sanzan or Three Kingdoms Period. The kingdoms had good ports and started to conduct active trading with China to increase their economic power.