Otsuboyama Kōgoishi (おつぼ山神籠石) was an ancient Korean-style fortress located in the Tachibana neighborhood of the city of Takeo, Saga Prefecture, Japan.

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.

[3] Otsuboyama Kōgoishi is located in a low hilly area extending from the mountain range to the west of the Saga Plain.

There are post holes at 3-meter intervals in front of the rows, which are thought to have been used to support boards for building the earthwork as they are tilted inward at a 10 degree angle.