How did secret shutters protect a lock’s keyhole in the Middle Ages?

In the Middle Ages, the security of locks relied primarily on elaborate warding rather than advanced internal mechanisms. To enhance this security and deter unauthorized access, locksmiths—notably those in Nürnberg—employed several clever techniques to obstruct potential lock pickers.

One such refinement was the use of secret shutters to conceal the keyhole, making it difficult for an intruder to locate the entry point for their tools. Additionally, locksmiths often incorporated blind keyholes into the design. These false openings served as decoys, forcing anyone attempting to pick the lock to waste significant time and effort on non-functional areas of the device.1