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 it being built on the foundations of the Roman Temple of Claudius. 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.

Colchester Castle was one of a generation of stone keeps built by the Normans in the aftermath of the Norman Conquest of England, as part of the Normans' effort to impress their authority upon the conquered Anglo-Saxons. Alongside the White Tower in London, Colchester is one of the earliest stone towers in England.

The attribution of the castle as a royal foundation is based on a charter of Henry I dated 1101, granting the town and castle of Colchester to Eudo Dapifer "as my father had them and my brother and myself", Henry's father and brother being William I, "William the Conqueror", and William II, "William Rufus". The somewhat unreliable Colchester Chronicle, written in the late 13th century, credits Eudo with the construction of the castle and gives a commencement date of 1076. The design of the castle has been associated with Gundulf of Rochester purely on the basis of the similarities between Colchester and the White Tower at the Tower of London; however, both keeps also resemble the much earlier example at Château d'Ivry-la-Bataille in Upper Normandy.

At one and a half times the size of the ground plan of the White Tower, Colchester's keep of 152 by 112 feet (46 m × 34 m) has the largest area of any medieval tower built in Britain or in Europe. The enormous size of the keep was dictated by the decision to use the masonry base or podium of the Temple of Claudius, built between AD 49 and 60, which was the largest Roman temple in Britain. The site is on high ground at the western end of the walled town and at the time of the Norman Conquest, a Saxon chapel and other buildings which may have constituted a royal vill lay close by the ruins of the temple. The obvious motive for reusing this site was the ready made foundations and the availability of Roman building materials in an area without any naturally occurring stone. Another factor may have been that the Normans like to see themselves as imperial successors to the Romans, William being frequently compared by his biographer, William of Poitiers, to Julius Caesar and his barons to the Roman Senate. The Colchester Chronicle described the temple site as a palace built by the mythical Roman-era King Coel; either way, it was providing a provenance for the Norman occupiers as the inheritors of a heroic past. Siting the castle so close to the centre of the town makes Colchester the exception to the rule that Norman castles were built as a part of the town's external defences, with access to open countryside.

The initial preparation of the site involved the demolition of surviving superstructure of the Roman temple, resulting in a layer of mortar rubble at the Norman ground level. The walls of the keep sit on narrow foundation trenches filled with rubble and mortar, and directly abut the edge of the Roman podium, except in the south where they are set back to avoid the original temple steps and to facilitate the digging of a well. The walls are made of coursed rubble, including septaria and Roman brick robbed from nearby ruins. Ashlar dressings are of Barnack and other stone, as well as Roman tile and brick. A large apse projects from the south-east corner, resembling St John's Chapel in the White Tower but there is no firm evidence that a similar chapel ever existed at Colchester. It has been speculated that an apse was added to the Temple of Claudius in the 4th century during a putative conversion to a Christian church and that the Normans followed this outline. The keep was divided internally by a wall running from north to south; a second dividing wall was added to the larger eastern section at a later date.

Initially, the keep was only built to the height of the first floor; remnants of the crenellations which surmounted this first phase can still be seen in the exterior walls. It seems likely that either a financial or military crisis dictated that the partly completed keep had to be made defensible. A Danish raid in support of the Revolt of the Earls in 1075 or the threatened invasion by Canute IV in 1085 have both have been suggested as possible causes. Another theory is that only a single-story structure was originally intended. The keep today has only two storeys; the original height is unknown because of demolition work carried out in the late 17th century. In 1882, J. Horace Round proposed that, like the White Tower, Colchester would have had four storeys, with a double-height great hall and chapel. This view was widely accepted throughout most of the 20th century. More recently, researchers have supported a three-storey model and some of the latest work suggests that there may have only ever been two storeys.

This is based on pre-demolition depictions of the castle, which despite errors and inconsistencies, all show the squat profile evident today rather than an immensely tall three- or four-storey tower, also the short time frame in which demolition can have occurred, and finally analysis of various surviving internal details which suggest that, unlike the White Tower, the great hall was on the first floor. Further uncertainty surrounds the position of the original entrance; the current main doorway in the southwestern tower dates from the second phase of construction which saw the addition of the first floor and staircases. Architectural features suggest that this second phase was undertaken after about 1100, probably by Eudo following the charter of 1101. In the mid-13th century, a masonry barbican was built adjacent to the south-west tower to protect the main doorway, replacing an earlier forebuilding.

The defences of the bailey consisted principally of a large earthen rampart and ditch surrounding the keep, the northern section of which survives but was heavily landscaped in the 19th century. Archaeological evidence has found that these embankments were thrown up over the remains of the Roman wall of the temple precinct and on the northern side were probably constructed at the same time as the first phase of the keep. The rampart to the north-east was 28.5 metres (94 ft) wide by 4 metres (13 ft) high. The southern embankment seems to have been completed during the second phase of keep construction around 1100. Inside the bailey, a late Anglo-Saxon chapel stood close to the southern edge of the keep and a domestic hall to the southeast of, and aligned with the chapel, were both retained during the first phase of keep construction.