Sakanoue no Tamuramaro held the title of Chinijufu-shogun, commander of the pacification forces, and he had assembled his army from ten eastern provinces stretching from Suruga to Shimotsuke. The campaign's objective was the submission of the Emishi, whose resistance had humiliated earlier imperial expeditions. Within weeks of Isawa Castle's construction, the Emishi leaders Aterui and More surrendered with some five hundred warriors. The captives were taken to the imperial capital in Kyoto, where Tamuramaro reportedly argued for mercy. The court overruled him. Aterui and More were beheaded at Moriyama in Kawachi Province. Today Aterui is remembered in Iwate as a hero of indigenous resistance, and the Oshu region commemorates both the general who conquered and the chieftain who defied an empire.

Isawa Castle became the anchor of imperial authority in the far north. A year after its founding, Shiwa Castle was established further north near present-day Morioka to serve as an administrative center. But that site proved prone to flooding and was abandoned by 811 AD in favor of Tokutan Castle ten kilometers to the south. Meanwhile, the garrison at Isawa shrank. By 815 AD it had been reduced to seven hundred men, many of them fushuu -- Emishi warriors who had entered imperial service. The government office complex at the castle's center, a separate enclosure measuring ninety meters on each side, processed tribute, administered justice, and projected authority across a landscape that remained more Emishi than Japanese. By the mid-ninth century, Isawa Castle fades from the written record, its role diminished as the frontier shifted further north.

Isawa Castle resurfaced in the historical record during the Zenkunen War of the 1050s and 1060s, when the powerful Abe clan -- descendants of established frontier families -- used the old fortification as a stronghold against the central government's forces led by Minamoto no Yoriyoshi. The castle's strategic value in the Kitakami valley endured long after its original garrison had dispersed. It may have finally been destroyed during the Gosannen War of 1083, another civil conflict among the warrior clans of the north. By then, the josaku system itself was obsolete, replaced by the personal power of local warlords who would eventually reshape Japan's entire political structure.

Designated a National Historic Site in 1922, Isawa Castle today is an archaeological landscape rather than a standing ruin. Excavations have revealed the precise geometry of the fortress: the double moats, the positions of gate structures, and the footprint of the central government compound. The site sits in the broad, fertile plain of the Kitakami River, surrounded by rice paddies that have replaced the dense forests Tamuramaro's soldiers would have encountered. Fifteen minutes by bus from Mizusawa Station on the JR East Tohoku Main Line, the grounds offer little verticality but enormous scale. Walking the perimeter of the outer earthworks, the sheer size of the enclosure becomes apparent -- this was not a watchtower or a guard post but a self-contained frontier city, built to hold an empire's ambitions at the very edge of its reach.