Colchester Castle is a Norman castle in Colchester, Essex, England, dating from the second half of the eleventh century. The keep of the castle is mostly intact and is the largest example of its kind anywhere in Europe, due to its being built on the foundations of the Roman Temple of Claudius, Colchester. The castle endured a three-month siege in 1216, but had fallen into disrepair by the seventeenth century when the curtain walls and some of the keep's upper parts were demolished; its original height is debated. The remaining structure was used as a prison and was partially restored as a large garden pavilion, but was purchased by Colchester Borough Council in 1922. The castle has since 1860 housed Colchester Museum, which has an important collection of Roman exhibits. It is a scheduled monument and a Grade I listed building.

The term Norman architecture is used to categorise styles of Romanesque architecture developed by the Normans in the various lands under their dominion or influence in the 11th and 12th centuries. In particular the term is traditionally used for English Romanesque architecture. The Normans introduced large numbers of castles and fortifications including Norman keeps, and at the same time monasteries, abbeys, churches and cathedrals, in a style characterised by the usual Romanesque rounded arches and especially massive proportions compared to other regional variations of the style.