In response, a huge network of shore fortifications was constructed throughout the rest of the 600s, often with the assistance of Baekje engineers, generals and artisans.

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.

Scholars after Tsuboi determined that the structures are most likely the remains of practical, military fortifications, and were unlikely to have significant spiritual connections, although much remains unknown about these structures and there is very little contemporary documentary evidence.Construction of Mizuki Castle began in 664 by order of Emperor Tenchi,[3] although in the Nihon Shoki it is described only "a large embankment in Tsukushi Province to store water".

The fortification was built on the soft ground of alluvial land with the Mikasa River flowing north through the center.

In 1975 the outer moat on the Hakata side of the fortification was discovered, and in 1978, Haji ware pottery with ink writing and the inscription "Mizuki" dating from the late 8th century was unearthed.