Unaware of the outbreak of the Silla-Tang War (670–676), the Japanese would continue to build fortifications until 701, even after finding out that Silla was no longer friendly with Tang.

In addition, the remains of two octagonal buildings and the discovery of a bronze standing Bodhisattva statue suggest that refugees from Baekje were involved.

The south side of the castle is located at a strategic point on a transportation route that is believed to have been an ancient official road.

[4] It is believed that the castle was renovated into a base for controlling northern Higo Province after the implementation of the Ritsuryō system at the end of the 7th century.

From the second half of the 8th century onwards, it is believed that it was transformed into a facility specialising in the storage of goods, with a number of warehouses organized into row.