Nagasaki was the port city of the Hizen Province (now Nagasaki Prefecture), and after 1570 began to prosper from the trade with Portugal. The government imposed a policy of seclusion on Japan in the early 17th century, and Nagasaki became the only port open to foreign trade. All the Dutch traders moved their commercial houses there in 1641. The piece of fan-shaped land that juts out into Nagasaki Bay, attached to the shore by a bridge, is the island of Deshima. Constructed in 1634, first home of the Portuguese, later the Dutch traders.