Nijo Castle, located in Kyoto, not Osaka, is famous for its "nightingale floors" (uguisubari). These floors were designed to chirp like a nightingale when walked upon, serving as a security measure to alert the occupants of any intruders. The intricate mechanism consists of metal clamps beneath the floorboards that squeak when pressure is applied, creating a distinctive sound reminiscent of a bird's song. This ingenious design helped protect the castle from potential sneak attacks.