The Sanctuary Knocker

The infamous sanctuary knocker. In medieval times,  it was an important symbol of the Cathedral's political role.
(Note: The current knocker is a replica - the original is displayed in the Cathedral Treasures Exhibit inside the building). 

The knocker on the Cathedral’s northern door, known as the Sanctuary Knocker, played an important part in the Cathedral’s history.  Those who ‘had committed a great offence,’ such as murder in self-defence or breaking out of prison, could rap the knocker, and would be given 37 days of sanctuary within which they could try to reconcile with their enemies or plan their escape.
Keeping Watch for the Seekers of Sanctuary
The Cathedral entrance has now been modified, but it originally had two small chambers above the doorway with windows where monks would be seated keeping a watch out for sanctuary seekers, to let them in promptly, at any time of the day or night. 
Announcing a Sanctuary Seeker’s Arrival
When somebody did seek sanctuary in the Cathedral, the Galilee bell would be rung to announce it. The sanctuary seeker would be given a black robe to wear, with St Cuthbert’s Cross sewn on the left shoulder to distinguish them as one who had been granted sanctuary by God and his saint.

https://www.durhamworldheritagesite.com/learn/architecture/cathedral/intro/sanctuary-knocker