Siege of Oshi Castle

**The Battle of Oshi Castle — The Failed Water Assault and the Legend of Kai-hime**

In Tensho 18 (1590), during Toyotomi Hideyoshi's campaign against Odawara, intense siege warfare unfolded at Oshi Castle in Musashi Province (present-day Gyōda City, Saitama Prefecture) between the Toyotomi forces led by Ishida Mitsunari and the besieged garrison of the Narita clan. This "Battle of Oshi Castle" is noted in Japanese history as a siege in which a garrison of merely some 500 defenders never surrendered to an army of tens of thousands, despite Mitsunari's bold strategy of constructing a large embankment called the "Ishida Embankment" to flood the castle.

Oshi Castle stood in the low-lying wetlands of the Tone and Ara river systems, surrounded by marshes, lakes, and paddies—a natural stronghold. This topographical character gave rise to its alternate name, the "Floating Castle" (Ukishiro). Because the castle occupied low ground surrounded by water, a conventional direct assault was difficult, and Mitsunari adopted a water-attack strategy, turning this apparent weakness to advantage. However, the terrain of the Tone River's floodplain proved equally challenging for those executing the water assault.

**The Narita Clan and Oshi Castle — Guardians of a Musashi Stronghold**

The Narita clan was a powerful regional daimyo based in northern Musashi Province, and Oshi Castle functioned as the Narita's principal castle throughout the Sengoku period. The castle lord Narita Ujinaga served under the Later Hōjō's Hōjō Ujimasa at Odawara, commanding one of the most significant powers in the Kantō region. However, when Hideyoshi launched his Odawara campaign in Tensho 18, Ujinaga himself was stationed at Odawara Castle, so the defense of Oshi fell to his designated administrators: first Narita Yasuki, and after his death, his son Narita Nagachika, who became supreme commander. Masaki Toshihide and Kai-hime also supported the defense.

Kai-hime is said to have been the daughter of Narita Ujinaga, and her martial prowess has been transmitted through later generations. She is often depicted as a female warrior skilled with both bow and naginata, credited with playing a commanding role in sustaining the garrison's morale during the siege. However, most surviving sources post-date her era, and primary documentation of Kai-hime's specific deeds remains limited. Nevertheless, her presence as a symbol of the Battle of Oshi Castle has been widely known since the Edo period.

**Ishida Mitsunari's Water Assault — A Failed Large-Scale Engineering Campaign**

In the fifth lunar month of Tensho 18 (June 1590), approximately 23,000 Toyotomi troops led by Ishida Mitsunari, Ōtani Yoshitsugu, and Natsuka Masaie surrounded Oshi Castle. Mitsunari immediately devised a water-attack strategy and began construction of a massive embankment—the Ishida Embankment—extending some 28 kilometers around the castle. This embankment was designed to divert water from the Tone and Ara rivers and inundate the castle.

The construction was completed in a short span of time, and water was directed around the castle. However, Oshi Castle proved to be built on higher ground than anticipated, and the defenders inside had fortified their positions against the rising water. Moreover, sections of the embankment broke under pressure from the diverted water during the assault, causing the strategy to unfold contrary to expectations.

**The Valiant Struggle of the Few — 500 Against 23,000**

The approximately 500 defenders who held the castle maintained unrelenting resistance despite facing an enemy force tens of times their number, employing bows, arquebuses, swords, and spears. The direct assaults that Mitsunari's forces carried out alongside the water attack were also repelled by fierce counterattacks from the defenders. During nighttime surprise raids launched from the castle (strikes from the rear approaches), the warriors of Oshi are said to have advanced with swords in hand against the enemy positions, inflicting casualties on Mitsunari's forces.

Particularly noteworthy are the repeated sorties commanded by Masaki Toshihide. During a period when Mitsunari's encirclement was incomplete, Toshihide led a small elite force out from the castle, disrupted the enemy's rear positions, and threatened their supply lines. This active defensive strategy sustained the morale of the garrison and enabled them to endure for an extended period.

**Oshi Castle and Swords — Musashi's Sword Culture and the Weapons of Siege**

Musashi Province has long been known as a center of sword production, and in the Edo period it would become the heart of the Edo new-sword tradition. During the Sengoku period, Bizen-style tachi and katana circulated widely in Musashi Province, and it is believed that the warriors of Oshi Castle carried high-quality blades.

Swords played a crucial role in Oshi's siege precisely in close-quarters combat under the unique conditions of water assault. Within a castle surrounded by water, where moisture rendered arquebuses unreliable, swords and spears functioned as the final practical weapons. In the night raids conducted by the defenders, close-range combat centered on the sword took place.

It is believed that the swords of warriors who fell inside the castle were divided between those returned after the castle's opening and those lost beneath the water. Even today, fragments of Sengoku-period blades and sword fittings are occasionally excavated near Oshi Castle, bearing witness to the intensity of the combat waged at this site.

**The Later Life of Kai-hime — Hideyoshi's Concubine and Tokugawa Ieyasu's Mediation**

After Oshi Castle was opened without falling, Kai-hime entered the Toyotomi household at Hideyoshi's request (some accounts say through coercion) and became one of Hideyoshi's concubines. It is widely held that "Tokugawa Ieyasu mediated the arrangement," and this is sometimes interpreted as a political gesture through which Ieyasu demonstrated consideration toward the Narita clan. After Hideyoshi's death, Kai-hime's fate receded from the historical record, but the legend of her martial deeds at Oshi Castle was transmitted through the common folk of the Kantō region during the Edo period.

In the modern era, Oshi Castle became widely known as the setting of the film "The Castle of Fools" (released in 2012), and the exploits of Narita Nagachika and Kai-hime drew renewed attention. The film's popularity led to increased tourism to Oshi Castle and Gyōda City, making the Battle of Oshi Castle a point of rediscovery for contemporary audiences.

**The Historical Significance of the Battle of Oshi Castle — Hideyoshi's Odawara Campaign and the Conquest of the Kantō**

The historical significance of the Battle of Oshi Castle is understood as one aspect of the conquest of the Kantō region in the final stage of Toyotomi Hideyoshi's unification of Japan. With the opening of Odawara Castle in the seventh lunar month of Tensho 18, the Later Hōjō perished and Hideyoshi's nationwide unification was completed. In the sense that Oshi Castle was the first to yield after Odawara's opening, it occupies a special place as "the castle that never fell."

Furthermore, Mitsunari's failure in the water assault is sometimes cited as one basis for critical assessment of his military capabilities. While his administrative acumen in conceiving the bold water-assault strategy and directing large-scale engineering works is acknowledged, the fact that he ultimately failed to capture the castle has been discussed among historians as evidence of his limitations as a military commander.