The Flooding Siege of Ōta Castle

Fought in Tenshō 13 (1585), the water siege of Ōta Castle in Kii Province (present-day Wakayama City) was the last major siege in Toyotomi Hideyoshi's Kishū campaign. Along with the sieges of Bitchū-Takamatsu Castle and Oshi Castle, it is counted among Japan's three great flooding sieges.

Start of the Kishū Campaign

Kii Province was a land where powerful self-governing armed forces held their ground: the Saika league, famed for its firearms, and the Negoro league, a group of warrior monks. Backed by the authority of temples and shrines and by their firearms expertise, they maintained independent power and repeatedly resisted central authority. In the 3rd month of Tenshō 13 (1585), Hideyoshi, advancing toward national unification, mobilized a force of about 100,000 under his younger brother Hidenaga as deputy commander and invaded Kishū. Hidenaga's forces took one hostile castle after another, burned down Negoro-ji, the headquarters of the Negoro league, and also brought the Saika league's strongholds under their control.

Ōta Castle That Would Not Fall

The last stronghold of Kishū resistance was Ōta Castle, held by the Ōta league. Ōta Castle was a strong castle surrounded by low-lying marshland, and it was not easily taken by direct assault. The garrison is said to have numbered about 3,000 to 5,000 men. Hideyoshi's side surrounded the castle with a large army said to number 60,000 to 100,000, but they were unable to make headway against its firm defenses.

A Vast Embankment Built in Just a Few Days

Hideyoshi, who had succeeded with the water siege of Bitchū-Takamatsu Castle in Tenshō 10, chose a water siege for Ōta Castle as well. To channel the water of the Kino River flowing around it, a vast embankment encircling the castle was built. Its total length is said to have reached about 7 km, and the embankment is said to have surpassed the one at Bitchū-Takamatsu in scale. Moreover, it is said that a total of some 460,000 laborers were mobilized, and that this vast embankment was completed in an astonishing period of just 5 to 6 days.

In Demizu, Wakayama City, part of the embankment still remains. It is about 24 meters wide and about 66 meters in surviving length, and its height is estimated at about 2.6 meters from the difference in elevation with the surrounding rice paddies. Archaeological excavations unearthed lead musket balls from the embankment soil, confirming that it was built in the Shokuhō period. This embankment remnant of the Ōta Castle water siege was designated a Wakayama Prefecture-designated cultural property in fiscal 2023, and it conveys the scale of the civil engineering work of the time to the present day.

Surrender and the Punishment of the Ringleaders

Ōta Castle, turned into a lake after the waters of the Kino River were diverted into it, nonetheless continued its stubborn resistance. But when the Toyotomi side deployed naval forces and attacked with firearms from boats, the castle commander Ōta Sakon and others, their provisions also exhausted, finally gave up resistance and, after about a month under siege, offered to surrender. While Hideyoshi permitted the lives of many of the besieged to be spared, he had the castle surrendered on condition that 53 ringleaders, headed by Ōta Sakon, commit suicide. The heads of the 53 men who were beheaded were displayed at Tennōji and Abeno in Osaka, and it is said that the punishment implicated women and children as well, which speaks to the severity of the water siege and the harshness of the postwar settlement.

The End of Medieval Armed Forces

This battle brought Kii Province under control, and the self-governing armed forces of Saika and Negoro, dating back to the medieval period, were dissolved. The fact that local powers that had wielded firearms and maintained their independence bowed before central authority symbolized a turning point of the age. For Hideyoshi's regime, which pressed ahead with the sword hunt and the separation of warriors and farmers, the capture of Ōta Castle was an event that embodied those policies. Those who had held their ground through military force were absorbed into a unified regime. The water siege of Ōta Castle reflects the tide of the times, as the age of warfare with swords and spears moved toward its end.