Trịnh–Nguyễn War
During the time of the Trịnh–Nguyễn War (1558–1775), Vietnam was divided into two countries: Dang Trong (South) and Dang Ngoai (North) with the Gianh River as frontier line. Đồng Hới was an important fortress of the southern Nguyễn lords. The Đồng Hới Wall (Vietnamese: "Thành Đồng Hới") was considered the barrier that protected the Nguyễn lords from the attack by the northern Trịnh family.