The Miyagawa clan, a local samurai family, built the first fortification on this site and called it Ushiya Castle. It was modest -- a frontier outpost on the flat alluvial plain between two rivers. But the Sengoku period transformed every strategic junction into a contested prize. Saito Dosan and Oda Nobuhide fought over the area as part of their larger struggle for Mino Province. Under Oda Nobunaga's consolidation, the castle passed to Ujiie Naotomo and then his son Ujiie Naomasa. When Hideyoshi took control, he recognized what the terrain offered: the castle's central area was small, but rings of water moats connected to the rivers encircled it, giving the fortress a total defensive footprint far larger than its core. Each successive general expanded the walls, deepened the moats, and strengthened the fortifications. By the 1590s, Ogaki was no longer a minor stockade. It was a layered water fortress guarding the most important pass in central Japan.

In 1600, Tokugawa Ieyasu moved west to seize power from the Toyotomi clan. Ishida Mitsunari, leading the Western Alliance loyal to Hideyoshi's young son Hideyori, drew his defensive line through Gifu Castle and Ogaki Castle, hoping to block the approaches to Osaka via both the Tokaido and Nakasendo highways. The plan fell apart immediately. Gifu Castle, held by Oda Nobunaga's grandson Oda Hidenobu, fell to Ieyasu's vanguard after just one day. At Ogaki, the castle's commander Ito Morimasa -- daimyo of a 30,000-koku domain and a committed Mitsunari loyalist -- proved an inept leader. Rather than exploiting the castle's formidable water defenses for a flanking attack as Ieyasu's forces passed, Mitsunari pulled his army to Sekigahara, ten kilometers to the west. Morimasa marched his garrison out to join them. The decision was catastrophic. After the Western Alliance crumbled at Sekigahara on October 21, 1600, Ogaki Castle fell to Tokugawa forces following a brief skirmish. The fortress that might have changed the battle's outcome was abandoned without a real fight.

The Tokugawa shogunate handed Ogaki Castle through a succession of fudai clans -- hereditary allies who had supported Ieyasu before Sekigahara -- until 1635, when Toda Ujikane received the castle and its domain. The Toda clan would hold it for the next 236 years, through the entire remaining Edo period until the Meiji Restoration in 1871. Under their stewardship, the castle was substantially repaired and improved. A four-story tenshu (main keep) rose above the walls, flanked by corner yagura watchtowers, the whole complex ringed by four concentric layers of moats. The Toda governed quietly. Ogaki Domain sat in the shadow of Sekigahara's fame, but the castle itself became one of the most complete flatland fortress complexes in the region, its water defenses fed by the same rivers that had made the location valuable since the Miyagawa clan first drove stakes into the riverbank.

When the Meiji government dismantled the feudal system after 1871, many of Ogaki Castle's outer structures were torn down and its outer moats filled in. But the central keep and its immediate defenses survived. In 1936, the tenshu and yagura were designated a National Treasure of Japan -- official recognition of the castle's role in the Battle of Sekigahara and its importance to the nation's history. That designation could not protect the keep from American firebombing during World War II. The original structures were destroyed. The current four-story tenshu is a 1959 concrete reconstruction, housing a museum of local history and samurai artifacts. The central grounds are preserved as a public park where cherry trees line the remaining inner moats. The castle sits low -- this is a flatlands fortress, not a mountain stronghold -- and from the air it reads as a pocket of old geometry amid the modern grid of Ogaki city. The moats still reflect the keep, as they have for four centuries, though the walls they mirror are newer than the water that surrounds them.