It first appears as Camulodunum, a 12-square-mile commercial and industrial centre named after the Celtic war god Camulos.
The key figure at this time was Cunobelin, known to the Romans as "King of the Britons" and to Shakespeare's audiences as Cymbeline. And it was his death, around ad 40, that suggested to the Emperor Claudius that the time was right for an invasion.
On landing in Kent in ad 43, a section of the Roman army made a beeline for Essex and the prosperous settlement on the River Colne. Claudius himself turned up in time to make a grand entrance, accompanied by elephants. For this was to be the first capital of Britannia.
On high ground overlooking the river, the legions built a fort whose plan survives in the modern town centre. But when, two years later, the soldiers departed for Wales, the town changed into civilian garb. It was now a colonia - a retirement town for former legionaries that would serve as a model of Roman life.
That meant theatres and baths, and a splendid temple to Claudius. But as the grand buildings went up, so the fort came down, leaving this juicy outpost of Rome ripe for the picking. And picked it was, when a few years later, Boudicca, queen of the Iceni, led her people against the occupiers.
Joining forces with the local Trinovantes, the rebels ambushed 1,500 soldiers marching to defend their capital. And from then on, it was a walkover. When the terrified inhabitants packed into the temple, the attackers burned it to the ground. Everyone was killed and the model Roman town was reduced to ashes. Then it was London's turn (70,000 died there), followed by Verulamium, the modern-day St Albans.
The Romans eventually crushed the rebellion, although nobody knows where. But the lesson was learned. Thereafter, important towns in Britain were surrounded by deep ditches and walls so strong that, in the case of Colchester, two-thirds of them remain to this day.