The stone wall supporting the keep was constructed by a technique called "ogi kobai" or "fan sloping", by which the upper part of the wall is curved outward like a fan. This wall is also called the Kiyomasa-style Crescent Stone Wall, after the general and engineer Katō Kiyomasa, who was in charge of its construction. The fan sloping technique was used to prevent swelling by curving the middle portion of the wall inward, thereby evenly balancing the stone weight against the pressure of sand and earth within.