Born in 1639, So Yoshizane was the third lord of the Tsushima-Fuchu domain. After a great fire in 1659 destroyed the castle town of Fuchu on Tsushima, including Kaneishi Castle, Yoshizane began rebuilding the town starting the following year, 1660. In 1665 he developed and expanded his residence into the form of a proper castle, which came to be known as Kaneishi Castle, and in 1669 he built a turret gate to serve in place of a main keep, giving the domain's seat of government suitable grandeur. Tsushima served as the gateway for diplomacy and trade with Korea, and under Yoshizane's rule the domain's key diplomatic functions -- including receiving Korean embassies -- continued to be centered on Kaneishi Castle. He died in 1702.

A steady ruler who, faced with the crisis of rebuilding after a great fire, methodically restored both the castle town and his residence, and who devoted himself to developing a base capable of bearing the domain's heavy responsibility for diplomacy with Korea.

The stone walls and turret gate that Yoshizane developed at Kaneishi Castle have been restored and preserved, and the site is now a National Historic Site adjacent to the Tsushima Museum, conveying the history of the domain's role as a center of diplomacy with Korea.