Before it became a castle of national importance, Koriyama was a minor fortification on the southern end of the Saikyo Hills, protected by the Akishino and Tomio Rivers. The site may have held a physic garden during the Nara period, with the first recorded fortification dating to the late tenth century. Its transformation began during the Sengoku period under Tsutsui Junkei, a local warlord who had previously commanded the sohei -- the armed monks of Kofuku-ji temple. Tsutsui Junkei seized the castle, lost it to the notorious Matsunaga Hisahide, then took it back. When Matsunaga's rebellion against Oda Nobunaga failed, Tsutsui Junkei was rewarded with control of all Yamato Province and chose Koriyama as his stronghold. His neutrality during the chaos following Nobunaga's assassination in 1582 earned him confirmation of his holdings under Toyotomi Hideyoshi. But loyalty bought only so much time -- after Tsutsui Junkei's death, his successor was transferred to remote Iga Province, and Koriyama passed to Hideyoshi's younger brother.

Toyotomi Hidenaga received Koriyama in 1585 and remade it to reflect his status as the second most powerful man in Japan. The castle's layout spiraled outward like a whirlpool: secondary and outer enclosures wrapped around a central bailey, which held a tenshu -- a castle tower -- in its northwest corner and the main gate at the southeast edge. The construction was massive, but Yamato's geology presented a problem. The province lacked the high-quality stone that other castle builders used. Hidenaga's solution was blunt: requisition everything. Twenty loads of stone per household. Sacred statues from temples. Gravestones. Even carved Buddhas from the Zoto, an eighth-century pyramidal Buddhist ruin in Nara. The resulting walls are an archaeological layer cake of repurposed sacred objects, visible to anyone who looks closely at the stonework today.

After Hidenaga's death, the castle passed to Nagamori Mashita, one of five magistrates governing for the Toyotomi regime. He improved the castle town and strengthened its water moats. But in 1600, when Japan's power brokers chose sides for the decisive Battle of Sekigahara, Nagamori backed the losing Western Army of Ishida Mitsunari. Tokugawa Ieyasu stripped him of his holdings and ordered many of Koriyama's defensive structures torn down. The castle's fortunes revived after the Siege of Osaka in 1614-1615, when Koriyama became the seat of a domain ruled by a succession of fudai daimyo loyal to the Tokugawa shogunate -- the Mizuno, Okudaira Matsudaira, and Honda clans each taking their turn. In 1724, Yanagisawa Yoshisato arrived from Kofu Castle, and his family would govern Koriyama for the next century and a half, all the way through to the Meiji Restoration.

Popular legend held that the tenshu collapsed in the 1596 Keicho-Fushimi earthquake because of a curse -- divine punishment for installing Buddhist statues upside down in its foundations. The story persisted for centuries, but contemporary documents tell a different tale: the castle tower was actually dismantled deliberately and relocated first to Nijo Castle in Kyoto, then to Yodo Castle. Archaeological excavations conducted between 2013 and 2017 confirmed that a tenshu definitely stood here during Hidenaga's time, settling an old debate. The foundations were real, even if the curse was not. The upside-down Buddha, however, is no legend -- it remains embedded in the castle walls, one of the most photographed oddities in Japanese castle architecture.

The Meiji Restoration dismantled Koriyama as thoroughly as any siege could have. All buildings except a single gate were destroyed or carted off to neighboring temples. Schools now occupy the outer enclosures. A Shinto shrine to the Yanagisawa clan stands in the central bailey. But the tall stone walls and deep moats survived, and citizens funded reconstructions of the Ote-mon Gate in 1983, the Ote-higashi-sumi-yagura turret in 1984, and the Ote-muki-yagura in 1987. The cherry trees planted when Yanagisawa Shrine was founded have earned Koriyama a place among Japan's "100 Best Cherry Blossom Spots," and the annual Castle Festival beginning April 1 draws crowds who wander the same walls where stolen Buddhas still hold their silent watch. Koriyama Castle was designated a National Historic Site in 2023 and listed among the Continued Top 100 Japanese Castles in 2017.