@Ki no jo is not appeared in the official history book, but later, it is in the documents such as Ki no jo Engi. According to that book, g a demon from other country came to Kibi. He was a prince of Kudara named Ura. Ura built a new residence on the Mt. Niiyama. He often robbed goods sent to the capital from southern country, and kidnapped women from the village around the mountain. Therefore, people were really afraid of him, and called his residence Ki no jyo (castle of a demon). People went to capital to report their terror.h This is the story generally called Ura folklore, and this place was named after his residence.
Ki no jo is a mountain shapes like an earthenware mortar turning down. Slope is really steep, but the summit is flat. Between eighth and ninth stage of this mountain is surrounded by 2.8km long castle wall. The wall is estimated to be about 7m wide and 6m high. This is how they made the castle wall. First, they lined up stones. Then, piled up the stones and they put dirt on between stones , and strengthen the wall by hitting the stone hard. It is not one long wall surrounding the building. The wall was divided into a few meter long unit to fit the landform. Area surrounded by the wall is about 30 hectare. It has four valleys. These valleys needed to be drained, so they had to make six water gates. There are also four entrance gates. There are food storage, cornerstone, place for signaling the fire, and reservoir. There are swamps to keep water. It is assumed that there used to be a army base and a workshop.