Château de Kozashijo, Castle site in Aizuwakamatsu, Fukushima Prefecture, Japan

Kozashijo was a flatland castle begun in 1600 in the Aizu Basin on the Aga River, with a circular layout of two enclosures: the honmaru measured 310 m east-west by 340 m north-south, and the ninomaru, including its moat, 710 m by 780 m over 55 hectares. Construction was halted in the summer heat and the castle was demolished unfinished in 1601.

Uesugi Kagekatsu began building Kozashijo in 1600 on the Kōzashi Plain beside the Aga River, with Naoe Kanetsugu as chief magistrate, because Wakamatsu Castle was too cramped. Some 120,000 labourers from four districts raised the honmaru stone walls between March and June; summer heat halted work and the Tokugawa campaign prevented completion. After Sekigahara, Kagekatsu was moved to Yonezawa in 1601 and the unfinished castle was demolished, its stones reportedly carried to Wakamatsu Castle.

The castle's name comes from the Kozashi plain by the Aga River, where the flatland site was laid out. The plan was a double-ring "回"-shaped flat castle, not a mountain fort. The keyaki tree at Takase, on the castle's demon gate side, still stands and is a national natural monument; tradition says the layout was oriented to the North Star.

Kozashijo lies in Kozashi-machi Honmaru, Aizuwakamatsu, on the flat floor of the Aizu basin near the Aga River, not on a hillside. Only earthworks remain: the honmaru measured 310 m east-west by 340 m north-south, the ninomaru 710 by 780 m including the moat, 55 hectares in all. The Takase keyaki, a national natural monument, stands at the castle's kimon, the northeast corner.

The plan was oriented using the Takase keyaki, a Japanese zelkova tree at the castle's unlucky northeast corner, and the North Star as reference points. The tree still stands and is a national natural monument. The site is now the Honmaru district of Kozashi town, Aizuwakamatsu.