Hailongtun is a mountain stone fortress built by the Bozhou Yang Tusi in the core area under its rule. It is surrounded on three sides by streams and one side by mountains. Historical records say that “birds and apes can hardly cross it.” Built atop the Longyan Mountain, Hailongtun was first built in the late Southern Song Dynasty when fierce battles were fought against the Mongolian army, and was almost reduced to ashes in the Pingbo Battle in the late Ming dynasty. From then on, it became desolate, leaving the dilapidated stone pass standing erect on the mountain ridge.