The Flooding Siege of Ōta Castle

The flooding siege of Ōta Castle in Kii Province (present-day Wakayama City) in Tenshō 13 (1585) was the last major siege of Toyotomi Hideyoshi's campaign to subjugate Kii, and it ranks with the sieges of Bitchū-Takamatsu and Oshi among Japan's "three great flooding sieges."

Kii was home to powerful autonomous armed leagues—the Saiga, famed for firearms, and the warrior-monk Negoro—who maintained independence behind temple authority and gunnery skill and had long resisted central power. In the third month of 1585 Hideyoshi mobilized some 100,000 troops under his brother Hidenaga and invaded Kii, reducing castle after castle, burning Negoro-ji, and crushing the Saiga strongholds. The last holdout was Ōta Castle, held by the Ōta league. Ringed by marshland, it proved too strong to storm; its garrison of perhaps 3,000–5,000, including non-combatants, held off a besieging force said to number 60,000–100,000.

Having succeeded with the flooding siege of Bitchū-Takamatsu in 1582, Hideyoshi chose the same method. To divert the Kii River, a vast embankment about 7 km long was raised around the castle—larger than the one at Takamatsu and reportedly built by some 460,000 laborers in only five or six days. Excavations have confirmed its remains, about 20 m wide at the base and 2.4 m high. When Hideyoshi's forces brought boats to fire guns across the flooded moat, the starving defenders gave in after about a month. Hideyoshi spared many but required the seppuku of Ōta Sakon and 52 ringleaders, whose heads were displayed at Tennōji-Abeno. The fall of Ōta pacified Kii and dissolved the medieval autonomous armed leagues—emblematic of Hideyoshi's drive toward the sword hunt and the separation of warrior and farmer.