Constructed over the massive vaults of the ruined Temple of Claudius, Colchester Castle represents the largest Norman Keep in Europe. Measuring some 152ft x 112ft (46 × 34m) and built largely from stone and brick quarried from the old Roman town of Colchester, the castle is believed to be the work of Gundulph, Bishop of Rochester. Gundulph was responsible for the design of the White Tower (the central Keep of the Tower of London) which is the only building in Britain with the same ground plan.
Work on Colchester Castle is thought to have been started c1076 and completed some 50 years later. The basic structure consists of a rectangular block with projecting towers at each corner, and a semi-circular apse on the south end of the east elevation. Colchester Castle originally stood four storeys high, but the upper two floors have since been lost.