“The Western Capital”, An Ancient International City With A Rich 1300-Year History, Registered As A Japan Heritage Site

The Nihon Shoki writes, “A large dyke will be built in Fukuoka to store water. It will be called Mizuki.” Characterizing the Mizuki Fortress ruins are earthworks roughly 1.2 kilometers long, 10 meters high, and 80 meters wide, made using engineering technology that was state-of-the-art for the time, such as rammed earth and fascines. In front of the earthen mound is an outer moat 60 meters wide and 4 meters deep. It is believed that wooden troughs (wooden water pipes) were buried within the earthworks, drawing water from the outer moat into storage. Fortress gates were built to the east and west of these earthworks, and the several buildings found inside the inner (southern) side of the east and west gate are assumed to be barracks and administrative offices monitoring traffic through the gates. The east and west gates were constructed with magnificent tiled roofs in the Nara Period, with the west gate used to welcome foreign envoys and the east gate used to welcome visitors from the capital. Meanwhile, this was also a place to bid farewell to travelers, and the east gate had a stage where farewell songs were performed for Otomo no Tabito and a woman named Kojima when returning to the capital. The earthworks were later repaired and the gates rebuilt to protect Dazaifu, but by the end of the Heian Period, their roles had been fulfilled.

The ruins of Kamiori Mizuki, Odoi Mizuki, and Tenjinyama Mizuki have been found on the west side of Mizuki, described as an “enormous dyke” in the Nihon Shoki, as though blocking the valley. Because these are small compared to the “enormous dyke”, they are called Shomizuki (“Small Water Fortresses”). While all are built with sturdy earthworks formed from many layers of earth, the method used to stack the earth differs slightly depending on location, meaning it appears that the construction method changed flexibly according to the terrain. An excavation of Odoi Mizuki Fortress found a wooden trough (wooden water pipe) that passed water under the earthworks, which used a civil engineering technique in which crossties were arranged to prevent the troughs from sinking as well as construction technology to connect boards together. These “small” Mizuki Fortresses built as a series of defenses along with Mizuki Fortress blocking the plains serve as civil engineering heritage that speaks to the high level of technology at that time.