Ito-jo Castle (怡土城)
Ito-jo Castle was a yamajiro (a castle built on a mountain to take advantage of the mountain's topography) which had been constructed on the west hillside halfway up Mt. Takasu on the boundary between Fukuoka City and Maebaru City, Fukuoka Prefecture, in the Nara period of the eighth century. It is a national historic site.
According to Shoku Nihongi (Chronicle of Japan Continued), KIBI no Makibi, who had joined the party of Kento-shi (Japanese envoy to Tang Dynasty China) twice to study abroad and then had become a high official in Dazaifu Seicho (local government office in the Kyushu region), started to construct a castle under the order of Emperor Koken in July, 756. Before completion of the castle, KIBI was replaced by SAEKI no Imaemishi because KIBI was put in charge of the construction of Todai-ji Temple, and the castle was said to have completed in February or March, 768 (February, in old lunar calendar).
In those days, in Tang 'An Shi Rebellion' broke out against Emperor Xuan Zong, in the Korean Peninsula Silla rejected the meeting with the national envoy from Japan, and the other events happened, so that the foreign strain was growing higher and the defense of Kyushu had become urgent.
It seems that the castle had the following characteristics to prepare against the attacks by the enemy: The castle was constructed in the shape of cross braces with the construction methods of the Chinese-style yamajiro; several watchtowers are placed along the ridge of the mountain; facing the flat ground at the west foot of the mountain, the earth and stone mounds which were 10 meters high were stretched into 2 kilometers from south to north to strengthen the defense; and castle gates and sluice gates were built between the both ends of the mounds. It is confirmed that there were also moats which were 15 meters wide outside of the earth mounds.
At present, as structural remnants, the earth mounds which are 1.6 kilometers long at the west foot of Mt. Takasu, and the traces of eight watchtowers in total on the ridge line have remained, but there are a lot of unknown points such as how the inside of the castle was utilized, and when the castle was abandoned.
The excavation in 2007 has made it clear that the moats in which pebbles were filled were placed inside the earth mounds. This is regarded as facilities to drain the water away so that the rainwater running down from the slope of the mountain might not burst the earth mounds, which is thought to be a unique device that cannot be found in the construction of yamajiro in the Chinese continent.
Later in the Sengoku period (period of warring states), the head of a local ruling family, Tanetsugu HARADA reused the castle to construct Takasu-jo Castle as a medieval yamajiro.