Colchester Castle Colchester Castle in Colchester , Essex , England , is an example of a largely complete Norman castle . It is a Grade I listed building . Construction . At one and a half times the size of the Tower of London ' s White Tower , Colchester ' s keep ( ) is the largest ever built in Britain and the largest surviving example in Europe . There has always been debate as to the original height of the castle . It has been suggested that the keep was at one time four storeys high , though for a number of reasons , including the peaceful region of the castle and the lack of local stone , it is now thought that it had only two or three . The castle is built on the foundations ( or the podium ) of the earlier Roman temple of Claudius ( built between AD 54 – 60 ) . These foundations , with their massive vaults , have since been uncovered and can be viewed today on a castle tour . The castle was ordered by William the Conqueror and designed by Gundulf , Bishop of Rochester . Building began between 1069 and 1076 under the supervision of Eudo Dapifer , who became the castle ' s steward on its completion . Building stopped in 1080 because of a threat of Viking invasion , but the castle was completed by around 1100 . Many materials , such as Roman brick and clay taken from the Roman town , were used in the building and these can easily be seen . Scaffolding pole holes and garderobe s can still be seen in the structure . Later history . In 1215 , the castle was besieged and eventually captured by King John , following the altercation with rebellious nobles that eventually led to the Magna Carta . The castle has had various uses since it ceased to be a royal castle . It has been a county prison , where in 1645 the self - styled Witchfinder General , Matthew Hopkins interrogated and imprisoned suspected witches . In 1648 , during the Second English Civil War , the Royalist leaders Sir Charles Lucas and Sir George Lisle were executed just to the rear of the castle . Local legend has it that grass will not grow on the spot on which they fell . A small obelisk now marks the point . In 1656 the Quaker James Parnell was martyred there . In 1650 a Parliament Survey condemned the building and valued the stone at five pounds . In 1683 an ironmonger , John Wheely , was licensed to pull it all down - presumably to use as building material in the town . After " great devastations " in which much of the upper structure was demolished using screws and gunpowder , he gave up when the operation became unprofitable . In 1727 the castle was bought by Mary Webster for her daughter Sarah , who was married to Charles Gray , the Member of Parliament for Colchester . To begin with , Gray leased out the keep to a local grain merchant and the east side was leased out to the county as a gaol . In the late 1740s Gray restored parts of the building , in particular the south font . He created a private park around the ruin and his summer house ( perched on the old Norman castle earthworks , in the shape of a Roman temple ) can still be seen . Charles Gray also added a library and a study . Twentieth and twenty - first centuries . In 1922 , the Castle and the associated parkland were given to the Town . The Park is split into the Upper and Lower Castle Parks . The Castle is now a public museum . Ownership . The later inheritance of the castle and its grounds is illustrated below . Only those greyed out did not at some time own the building . Though Charles Gray Round died before the area was sold to the corporation of Colchester , his will ensured that it was held in trust with that eventual purpose .