In the middle ages, someone accused of a serious offence could claim sanctuary in Durham Cathedral by knocking on the door. Having knocked and claiming sanctuary the local constables were unable to take any further action in the precincts of the Cathedral, such was the power of the Prince Bishops.
The fugitive from justice then had a period of thirty seven days in which they had to choose whether to stand trial in front of a magistrate or judge, or whether to leave the country by the nearest port.
Whilst the fearsome looking knocker on the north door looks really old, it is in fact a replica of the original. The original knocker remains on display within the cathedral in the Treasures of St Cuthbert exhibit.