The geography of Saijō underwent great changes during the Edo period. The second Hitotsuyanagi ruler, Naoshige, built Saijō Castle and diverted the Kita River to create a moat. The former castle grounds now house Saijō High School, one of a few public schools in Japan contained within a moat and castle gate. Flood control projects for the Kamo River and land reclamation from the Seto Inland Sea began during the end of the Warring States Period and continued through the Edo Period, with major reclamation projects undertaken by the Matsudaira family as a means to increase their personal land holdings, 20000 koku of which were lost when they were reassigned to Saijō.