A famous feature of nijoujou
, the Tokugawa shogun
s' castle in Kyoto
, Japan. The wooden floorboards of the walkways surrounding all the rooms (traditional Japanese, tatami
-covered) are rigged so that they rub against little metal pegs when people walk over them. This produces a slight squeaking sound. When several people walk over them at the same time, it really does sound a lot like a singing bird.
The point of all this was to defend against assassins by making it hard or impossible to sneak about soundlessly in the castle.