Hiuchiyama (火打ち山) was one of Minamoto no Yoshinaka's fortresses in Echizen Province, Japan. In April and May 1183, a Taira force led by Taira no Koremori attacked the fortress.
It was built on rocky crags, and well-defended; the Minamoto had even built a dam to create a moat. However, a traitor within the fortress tied a message to an arrow, firing it into the Taira camp, and revealing a way to breach the dam and drain the water. The castle soon fell to the Taira, but Yoshinaka and much of his forces survived and escaped.
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Text from the Wikipedia article “Siege of Hiuchi”, available under CC BY-SA 4.0.