Osaka Castle is a three tiered fortress at Osaka.
Who ever controls Osaka Castle controls the heart of the country.
It was originally ruled by Satoshi. However it was put under siege, by Daimyo Kamakura's army, during "The Battle of Osaka Castle." Satoshi's forces eventually lost the castle, when ninja scaled the walls, and captured it. Students of the Niten Ichi Ryu, including Jack and his friends, defended the castle and fought with Satoshi's forces, before eventually being defeated.