In the period immediately preceding the Roman invasion of Britain the Catevellauni tribe occupied the area of Britain now taken up by Northamptonshire, Bedfordshire, Cambridgeshire, Buckinghamshire, and Hertfordshire. The "capital" of the Catevellauni may have been at Wheathampstead, near St Albans, and by the time of the Claudian invasion, the southern border may well have been the Thames River.
When Julius Caesar invaded Britain in 54 BC he did battle with Cassivellaunus, who may have been the king of the Catevellauni. Roman reports put the army of the Catevellauni at 4000 war chariots, which certainly suggests that the tribe was both numerous and able to muster a powerful military force. Caesar defeated Cassivellaunus but withdrew after extracting a promise of submission.
Subsequent kings included Tasciovanus and Cunobelinus (Shakespeare's Cymbeline), who seems to have moved the tribal capital to Colchester. Cunobelinus was succeeded around AD 40 by his sons Togodumnus and Caratacus, who led the British resistance to the Roman invasion under Claudius in AD 43.
The Catevellauni had managed to expand their territory and influence, in the process arousing the antagonism of their Celtic neighbours. Fear and resentment of Catevellaunian power drove other tribes into an alliance with the Romans. For that reason, the Romans were able to isolate and defeat the Catevellauni with comparative ease.
Caratacus fled to Wales, where he continued the fight against Rome. Eventually, he was captured and taken to Rome, where he apparently was pardoned and lived out his life.
NOTE:
The name of this Celtic tribe is variously spelled Catevellauni and Catuvellauni. It should be noted that the names of the British Celtic tribes are those assigned by Roman commentators, and not necessarily those employed by the Celts themselves.
