The emperor Claudius began his conquest of Britain in AD 43 and his first military target was Camolodunun, the tribal capital of the Trinovantes. The invasion was mounted by four legions accompanied by auxiliary units under the command of Aulus Plautius. Claudius led the final military advance into Camolodunun, and received the submission of several tribal leaders here, thus securing his own position as head of the Roman empire.
Britain's first legionary fortress was constructed on the highest ground in Camolodunun, within the defences of the Iron Age settlement, in the same location as the present-day town centre. It became home to Legio XX which inhabited the fortress for five or six years, between AD 43 and AD 49, until the advancing Roman frontier required that this legion move on to Kingsholm, Gloucester.
There is considerable archaeological evidence which relates to the legionary fortress at Colchester. The defensive ditch and rampart of the fortress and the ditch and rampart of the annexe have been recorded during excavations at Lion Walk, Long Wyre Street, Balkerne Gate and Lion Walk United Reformed Church. The purpose of the annexe is unknown but it may have included a parade ground or legionary baths. The streets of the fortress have been recorded in several locations across the town including the via principalis (the main north-south street) which was investigated at Culver Street and at the Telephone Exchange Site. Legionary barracks have been located at numerous sites in the town, although none has been completely excavated. It is estimated that there were at least 60 barracks within the fortress, with each one designed to accommodate a century of soldiers and their centurion. Other military buildings have been identified at Culver Street, Lion Walk and at the Telephone Exchange site.