Ninjas were known as stealthy spies and assassins who could carry out any number of missions, from theft to assassinations. In 16th-century Japan, tales of ninja prowess spread, leading to innovative defenses by the 17th century. Castle builders created anti-ninja floors to alert residents of intruders. They did this by placing metal brackets with nails beneath the joists, which would flex and produce a bird-like noise when stepped on. This clever design helped keep ninjas at bay and ensured the