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 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.

[10] 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.

[11] 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.

[16] 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.

[16] 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.

[14] 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.

In November of that year, John arrived at Colchester, probably in an unsuccessful attempt to win over Lanvalai, who shortly afterwards left the castle in the care of the sheriff and joined the other rebel barons at Bury St Edmunds.

The barons later marched on London, forcing John to accept the Magna Carta at Runnymede in June 1215, which included a provision that Colchester be returned to Lanvalai.

[23][page needed] John besieged Rochester Castle before sending an army towards Colchester, under the command of a French mercenary called Savary de Meuleon.

The siege began in January 1216 and ended in March when King John himself arrived; the French garrison of 116 men were able to negotiate a safe passage to London.

[23][page needed] Following the capture of Colchester, Harengood was reinstated as constable and made sheriff, but in 1217, the castle was handed-over to the French and the barons as part of a truce agreement.

However, it was recovered by the boy king Henry III in the Treaty of Lambeth in September 1217 which finally ended the war, and William of Sainte-Mère-Église, the Bishop of London, was made constable.

She is believed to have planted the sycamore tree which is still growing on top of the southwest tower, either to celebrate the Battle of Waterloo in 1815 or to mark her father's death in the same year.

Between 1920 and 1922, the Castle and the associated parkland were bought by the Borough of Colchester using a large donation from Weetman Pearson, 1st Viscount Cowdray, a wealthy industrialist who had been the town's Member of Parliament.