Nightingale Floors: The World's First Security System
The Ninomaru Palace is famous for its 'uguisubari' — Nightingale Floors that squeak with every step, making silent movement impossible. The floors were engineered deliberately: metal clamps beneath the boards create the distinctive bird-like chirping sound, so that no assassin could ever approach the Tokugawa shogun's sleeping quarters undetected. The floors still work today, and walking through the palace you can hear every step announce itself.