When I went to Fuji’s Jukai on November 2, 2018, I found a stone wall like a castle wall in the forest of Jukai.
The country of Kai is the area of Yamanashi prefecture now, Motosu close to the border of the country of Kau Kai at that time Nakamichi Highway, there was a road Nakamichi, it seems there was a main castle as a key point to protected it.