The rectangular "Ni-no-maru" measures 135 metres (443 ft) from east to west, 150 metres (492 ft) from north to south, and 15–17 metres (49–56 ft.) above sea level. A map made in the early 17th century shows a residence bounded by "yaguras", walls, and "nagayas" (long structures). Another illustrated map of the castle made during the reign of the third lord, Kuroda Mitsuyuki, has a note about this "kuruwa" which says, "Ni-no-maru, the residence of Hizen-no-kami." Considering that Hizen-no-kami (an honorary title which meant "governor of Hizen") was the title given to the heir of Mitsuyuki, this compound was used as the residence of the heir to the lordship, at least when the map was made in the late 17th century. In addition, another old document indicates that the residence of Mitsuyuki, then the heir of Lord Tadayuki, was in the Ni-no-maru, while yet another document says that during the reign of the fifth lord Nobumasa, his uncle Nagakiyo, the lord of the county of Nogata, had stayed in the Ni-no-maru when he visited Fukuoka. The "kuruwa", which is mentioned in the two latter documents, probably refers to the Ni-no-maru.