Though covered in moss, the material of the rampart is lava that flowed from Mount Fuji and then hardened. It was skillfully constructed by piling up this lava rock.

If you look closely, one side of the rampart is stacked in an almost vertical form. By contrast, the opposite side has a slight slope.

It is thought that the nearly vertical side was the side facing the “threat” which was meant to be repelled.

That would be the outer side of the rampart arranged in an arc.