On the assumption that the description of the "Nihonshoki" (Chronicles of Japan) is correct, and based upon common sense, Miyakonojo which served as the central function, was constructed and developed enough for materials to be applied before 664 when many materials were put to construct affiliated facilities including mizu-shiro (castles on lakes or marshes for defensive purposes) for defense.
『日本書紀』の記述が正しいとして、常識的に考えれば、多くの資材を投入して防衛のための付属施設である水城等が建設されたとされる664年には、既に本体である都城は存在し、資材を投入するに足りる発展を遂げていたと考えられる。