This is the ruins of the bulwark built in 1276 to prepare against Mongolian invasion. It stretches from Nagatare Coast to Odo Coast for about 2.5 kilometers. It was found in an archeological research that the bulwark was built by piling up stones into the wall of 1 - 1.5 meters in width and 1.8 meters in height and the inside the wall was reinforced with clay.

The famous scene from the picture record of Mongolian invasion (Moko Shurai Ekotoba) depicts a historical scene of this very place, featuring Takesaki Suenaga riding a horse, who served as a vassal of Shogun at the time. Today, a part of the bulwarks is maintained and open to the visitors.