The castle's origins reach back to the Muromachi period, when Shiba Kaneyori claimed lordship over Dewa Province and built a fortified residence on a site that would become the inner bailey. He changed his surname to Mogami, and the clan he founded held the surrounding territory for some 275 years. But by the Sengoku period, internal feuds and short-lived leaders had eroded their strength. The aggressive Date clan invaded, reducing the Mogami to vassals. Only when the Date themselves stumbled into political crisis did Mogami Yoshimori seize his chance, reclaiming independence and cementing an alliance by marrying his daughter to Date Terumune. Their grandson, Date Masamune -- one of the most famous warlords in Japanese history -- was the product of this strategic union. The Mogami had traded a daughter for survival.

It was Mogami Yoshiaki, Yoshimori's eldest son, who transformed Yamagata from a fortified residence into a true castle. In 1592 he added the second and third baileys, erected two- and three-story yagura watchtowers, and laid out the surrounding castle town that would become the city of Yamagata. But Yoshiaki's relationship with the ruling Toyotomi regime was poisoned by a personal tragedy: Toyotomi Hideyoshi executed Yoshiaki's daughter Komahime during a purge of Toyotomi Hidetsugu's household. When the Sekigahara campaign erupted in 1600, Yoshiaki needed no persuasion to side with Tokugawa Ieyasu. Despite being severely outnumbered by an invading Uesugi force under Naoe Kanetsugu, the Mogami held their ground. Their reward was enormous: 570,000 koku of territory and recovery of the Shonai region, prompting a further expansion of the castle.

The triumph lasted eight years. After Yoshiaki's death in 1614, his retainers fell into civil war, and the Tokugawa shogunate seized the pretext to strip the Mogami of their domain in 1622. The castle passed to Torii Tadamune, then cascaded through a parade of daimyo clans -- sometimes changing hands within a single generation -- each one poorer than the last. With revenues slashed, these lords could not maintain an enormous castle built for a 570,000-koku domain. By the mid-Edo period, the main bailey had crumbled to ruins. The second bailey served as the daimyo's modest residence. The western half of the third bailey was plowed into farmland. A fortress that once dwarfed every castle in northern Honshu was slowly consumed by rice paddies.

The Meiji Restoration brought another metamorphosis. In 1872 the castle grounds became a base for the Imperial Japanese Army's 32nd Infantry Regiment. In 1906, some 1,500 cherry trees were planted along the old moats and ramparts to commemorate the Russo-Japanese War, and today Kajo Park -- as the grounds are now known -- is one of Yamagata Prefecture's premier hanami destinations, with blossoms peaking in mid-April. After World War II, the site was converted into a public park and home of the Yamagata Prefectural Museum. Restoration work has been steady: the East Gate was rebuilt in 1986, the Higashi Otemon Gate in 1991, the Inchimon Gate stonework in 2004, and a bridge to the main bailey in 2006. Yamagata City aims to restore the castle to its early Edo-period appearance by 2033. In 2006, it was recognized as one of Japan's 100 Fine Castles.