Ōnojō Fortress Ruins 大野城跡

The 663 loss spurred rulers in the 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 Mizuki Fortress, a 1.2-kilometer-long fortified embankment between Hakata Bay and Dazaifu, as well as a second mountaintop fortification known as Kiijō Fortress.

Ōnojō Fortress was constructed using the techniques and guidance of exiled nobles from Baekje, who patterned it after mountain fortresses in their own capital. Workers first flattened the ground, dug a pit, and erected wooden walls around the section to be filled. The planks retained the earth and sediment that was then poured in and tamped down, layer by layer. Most of the walls surrounding the fortress were also built with this hard-packed earth, but sections of stone wall between 3 and 8 meters in height guarded the valleys. Often springs or small rivers flowed through these valleys, so the walls had culverts or gaps in the rock to permit water flow. Excavators have found or uncovered 9 gates, some 70 buildings (primarily storehouses from the eighth century and later), and a central structure thought to have been a headquarters.

Ultimately there was no invasion, but Ōnojō Fortress was kept manned until around the tenth century. Its construction represented one of Japan’s earliest efforts at a wide-reaching national defense policy. In addition, the use of continental building techniques to construct Ōnojō Fortress is just one of the many signs of cultural and technological exchange occurring in Dazaifu at that time.

Built in 665, this immense walled fortress once ringed the summit of Mt. Shiōji, north of Dazaifu. Along with a smaller fortification to the south, Ōnojō Fortress is Japan’s oldest known mountain fortress. More than 1,350 years after its construction, large sections of the perimeter wall remain intact.

Ōnojō Fortress was conceived as one of several fortifications intended to guard the Hakata Bay area. In 663, the remnants of the Korean kingdom of Baekje (? CE–660 CE) and its Japanese allies had been defeated by the combined forces of Tang-dynasty China (618–907) and Silla (? CE–935 CE), another Korean kingdom. The loss spurred the Japanese imperial court, which was fearful of an invasion by rapidly expanding Tang, to order the construction of Mizuki Fortress in 664, then the mountain fortresses Ōnojō and Kiijō in 665. Ōnojō Fortress was a vantage point from which to observe enemy ships heading toward Hakata Bay.

Unlike many later castles, Ōnojō Fortress never had a tall central keep or series of moats. At its center was a camp encircled by 8 kilometers of earthen and stone walls. Exiled nobles from Baekje oversaw the construction, patterning Ōnojō Fortress after mountain fortresses on the Korean Peninsula. They introduced construction techniques that allowed for the creation of stone walls up to 8 meters in height. Where Ōnojō’s perimeter wall ran along ridgetops, it was built of rammed earth, but sections in the valleys were made of stone. Researchers have discovered the ruins of 9 gates and over 70 buildings, including storehouses and administrative headquarters in the central camp. Ultimately there was no invasion, but Ōnojō Fortress was manned and functional until around the tenth century.

Many sections of both the earthen embankments and the stone walls survive. To see them, visitors can take one of the hiking trails from Ōnojō Sōgō Kōen (Madoka Park) or drive to the Shiōji Kenmin no Mori Center on the mountain and walk from there.