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Canterbury Castle was one of the first castles to be built by William of Normandy after his victory at the Battle of Hastings in 1066. The first castle was probably not particularly sturdy, but during William II's reign (1087-1100) the stone keep that we see today was started and was completed during Henry I's reign (1100-1135).
However by the end of the C12th, this little castle's importance was reduced due to the massive fortifications of Henry II's extensions to the castle at Dover.
So Canterbury Castle ceased to be a defensive or military structure and became a prison for the area. In the C13th a gatehouse and large doorway were built on the SE side of the castle to provide better access to the prison. Only essential repairs were carried out, and by 1390 the structure was said to be "in every part weak and ruinous".
Comparatively expensive repairs were carried out but it seems that the prison was fairly insecure as during the mediaeval period up to 1548 the people of the town passed freely through its yard on their way to Wincheap.
By 1609 the castle was apparently ruinous again, and James I granted it to Sir Anthony Weldon.
The castle was partly demolished in 1770 and in 1792 much of the outer bailey wall was demolished to improve the line of Castle Street.
In 1825 the castle suffered the ignominy of being bought by the newly created Canterbury Gas Light and Coke Company. This company removed all the internal floors and used the shell of the castle as a coal and coke store !
The poor old castle was finally saved for posterity by the City of Canterbury who bought it in 1928 in order to preserve this bit of history.