Inabayama Castle

Inabayama Castle, later Gifu Castle, is a castle initially created by Saitou Dousan to be nearly impenetrable as a defensive position. It is easily defended and very difficult to attack.

Details
Because the castle is located on a mountain it has a strong natural defence, several towns surrounding it can be transformed into military positions immediately due to the system Dousan put into place, making it impossible to attack the castle from all sides and yet very easy to protect it as well. The mountain passes are also ideal for ambushes and tactics of the sort, it's known that a swamp is located somewhere nearby.

The mountain it stands on is called Mount Inaba

Arrival in the Sengoku Period Arc
While itself not present, Inabayama castle and hanbei are directly noted to be the major reasons later on why Saitou Dousan was unable to stop Yoshitatsu's rebellion.

Invasion of Mino Arc
The castle functions as Yoshitatsu's base of operations throughout the arc, with Hanbei's strategies protecting it with her strategies two Oda invasions are repelled before their troops even get close to the castle itself.