Osaka Castle - Sanadamaru
The Sanadamaru was made famous by the 2016 Taiga Drama of the same name. The Sanadamaru was a a satellite fortification built by Sanada Yukimura just south of Osaka Castle to protect it from Tokugawa's attacks in 1615. There are no remains to see but there are some signs around the site explaining what it was and where. There is also an interesting "Sanada Escape Tunnel" at the nearby Sanko Shrine. Unfortunately, there is no evidence that such a tunnel was built by Sanada. It was more likely a plan by Tokugawa to undermine the walls of the Sanadamaru. Other similar tunnels were also found but reburied for safety. The actual location and shape of the Sanadamaru was somewhat of a mystery until a new map was found in 2016.