Nagahama Castle, Japanese water castle in Kōenchō, Japan

Nagahama Castle is a water fortress near Lake Biwa with a three-story concrete tower topped by an observation platform. Inside, the building houses exhibits about the region and its development during the feudal era.

Toyotomi Hideyoshi ordered the original fortress built between 1575 and 1576 as a military stronghold. The fort was dismantled in 1615 and its stones repurposed to construct Hikone Castle.

The castle's name comes from a character borrowed from Oda Nobunaga: Hideyoshi renamed the area Imahama as Nagahama. Two legends tell of human pillars, Okane and Okiku, buried during construction; a stone monument to Okane stands at the keep site and a small shrine honours Okiku. The castle town was moved wholesale from Odani, and traces of the Hashiba period remain in its streets.

Visitors reach the castle with a 7-minute walk from Nagahama Station through the town streets. Entry to the museum requires a ticket and the observation deck offers open views of the lake and surrounding mountains.

Two legends explain the name Okane Moat, on the north side of the castle tower: during the 1574 construction a woman named Okane, said to be the most beautiful in Nagahama, was buried there as a human pillar. A stone monument at the tower site tells her story, and a small shrine nearby honours Okiku, who volunteered as a pillar to save her blind sister Shinobu.