Mizuki Fortress Ruins
Mizuki Fortress was a 1.2-kilometer-long, 80-meter-wide defensive embankment built in 664. The imperial court ordered its construction to guard the Dazaifu area—and ultimately, all of Kyushu—against a potential invasion by China’s Tang dynasty (618–907). The massive wall was built to connect the ridgelines of two mountains south of Hakata Bay, thereby closing off the large plain that opened up farther south. The walls of Mizuki Fortress stood around 10 meters high. In front of the embankment, facing the bay, was a 60-meter-wide moat that exceeded the maximum range of archers of the day.
In 663, Japan and the remnants of the Korean kingdom of Baekje (? CE–660 CE) suffered a disastrous military defeat at the hands of China’s Tang dynasty and Silla (? CE–935 CE), another Korean kingdom. The loss spurred the imperial court to create large-scale fortifications south of Hakata Bay, which seemed the most likely landing place for foreign ships. This round of defensive infrastructure also saw the building of two mountain fortresses in 665, located on strategic peaks overlooking the plain: Ōnojō and Kiijō Fortresses.
Mizuki Fortress was constructed using techniques brought from the Korean Peninsula. Groundwater and rivers such as the Mikasa River, which flowed through the center of the embankment site, made the terrain swampy. This meant that the base of the wall would sit upon soft, waterlogged soil that would not provide firm support. The ancient builders had a solution, however—they filled the lowest layers of earth with leaves and branches to prevent the embankment from sinking. Once a stable base was established, they erected wooden walls around the section to be filled. The planks retained the earth and sediment that was poured in and then tamped down, layer by layer. Each layer was firmly packed before the next was added.
In the base of the wall, wooden culverts were put in place. On the south side, within the fortress, water from the surrounding area was diverted to fill a series of artificial ponds; it flowed from the ponds through the culverts, beneath the massive embankment, and into the moat. This clever use of water as a form of defense gave the fortress its name: “Mizuki” means “water castle.” The fortress had two gates, one in the east and one in the west. The eastern gate has never been excavated, but the walls on either side of the western gate were particularly fortified, built of stone blocks rather than earth.
There was no invasion from the continent, but Mizuki Fortress went on to serve a role in amicable diplomacy. In the early eighth century, the city of Dazaifu was built to the south, and the fortress acted as its northern wall. Foreign dignitaries visited Dazaifu before continuing east to the imperial capital, and they entered the city through the Mizuki Fortress’s west gate. Accordingly, all of the diplomats, imports, and culture that flowed into Dazaifu—and from there to the rest of the country—came through this gate. In this way, the fortress became the literal gateway to Japan, a role it fulfilled until the end of the eighth century, long after its defensive function had ended. As ports and marine trade grew in importance, however, the focus of activity shifted northward toward Hakata Bay.
Much of this ancient fortress still stands, although some sections are split by modern roads and railways. It can be seen up close in several local parks, or as a long line of trees visible from overlooks on Mt. Tenpai and Mt. Shiōji.
