Guildford Castle was raised shortly after the Norman invasion by William I as one of a chain of fortifications intended to secure control of London. It was rebuilt in stone during the twelfth century including construction of a Great Keep intended to serve as a Royal residence. By the thirteenth century the castle was being used as the county gaol and continued in this role until 1544.

Source: http://www.castlesfortsbattles.co.uk/south_east/guildford_castle.html