Theory 1: A defensive facility from the Sengoku period
Even today, the area called Motosu lies on the border with Shizuoka Prefecture, and from long ago it was an important point marking the boundary between Suruga and Kai. Inside the arc-shaped stone rampart was the Motosu settlement.
For that reason, one idea is that it was built to defend this settlement, a key point on the road, from invasion coming from Suruga.
