Rokuban-yagura Turret

At one time there were seven turrets serving as the castle defenses. Only the Rokuban-yagura (sixth) Turret and the Ichiban-yagura (first) Turret remain today. There is a hole called the gmysterious passage of Osaka Castleh at this turret. What lies at its interiorc?
It is a National Important Cultural Property.