Mizuki Fortress Ruins
The Shōmizuki (meaning “little Mizuki”) Fortresses were earthen fortifications built at roughly the same time as the main Mizuki Fortress, a 1.2-kilometer-long earthen embankment constructed in 664. In combination, the structures were intended to act as a giant wall preventing foreign armies from entering the plain that lay south of Hakata Bay, where Dazaifu would later be constructed.
The smaller-scale Mizuki Fortresses were built in the foothills west of the main Mizuki Fortress, and in comparison they were lower in height, shorter in length, and thinner in depth. Their function was to plug low-lying river valleys that might give access to an army trying to bypass the main Mizuki Fortress by going west. Today, the earth has been cleared and leveled by later developments, but in the seventh century the smaller Mizuki Fortresses were bordered on both ends by high valley walls.
Ōdoi Mizuki Fortress
Ōdoi Mizuki Fortress was built perpendicular to a river, with natural high ground rising on either side of the embankment. It was 110 meters long, with walls around 7 meters high. The north side, facing Hakata Bay, had a 40-meter-wide moat. Because of structural similarities in the water channels of this fortress and those of the main Mizuki Fortress, it is likely that both embankments were constructed as part of a unified plan of defense.
Tenjinyama Mizuki Fortress
Tenjinyama Mizuki Fortress was built the farthest from central Dazaifu, in what is today a residential neighborhood in the city of Kasuga. The fortress had walls 140-meters long and 5 meters high, but the width of its moat is unknown. The layered-earth construction of the walls left strata in the soil, and in some places these layers are exposed and easily visible from the street.
