CASSIVELLAUNUS (fl. 54 B.C.), a British prince contemporary with Julius Cæsar, whose territory lay to the north and north-east of the river Thames, comprising roughly the modern counties of Hertfordshire, Buckinghamshire, and Berkshire; its exact limits are uncertain. The people over whom he ruled were the Catuvellauni, a powerful and warlike nation who had encroached upon the surrounding tribes; their territory had been much extended before Cæsar's arrival in Britain by Cassivellaunus, who had been engaged in constant conflicts with his neighbours, and his conquests had given him such supremacy over them that he was recognised as their natural and undisputed leader against the invader. Cassivellaunus is first mentioned by Cæsar in his account of his second expedition to Britain in the summer of 54 B.C. Cæsar relates how, after having effected a landing and advanced some twelve miles into the interior of the country, he was recalled to the coast by the intelligence of the destruction of the greater part of his fleet in a storm. Ten days were consumed in repairing the ships that remained, and then, advancing to the Thames, Cæsar found the enemy drawn up in great force on the northern bank of that river, under the command of Cassivellaunus. In spite of the British fortification of the banks, the Roman soldiers crossed the river, and the Britons were unable to stand before their attack, but the progress of the Romans was much impeded by the skilful use made by Cassivellaunus of his charioteers, four thousand of whom were employed in harassing Cæsar's line of march. In the meantime the Trinovantes, another powerful people, occupying what is now Essex, and part of Middlesex, sent envoys to Cæsar to announce their submission. Mandubratius, the son of their former king Imanuentius, had fled for refuge to Cæsar, in order to escape the fate of his father, who had been killed by Cassivellaunus in the course of his conquests over his neighbours. The Trinovantes asked Cæsar to send Mandubratius to rule over them and to protect him from Cassivellaunus. Cæsar granted their request, and sent Mandubratius to them, at the same time demanding and obtaining hostages and corn. The example of the Trinovantes was speedily followed by other tribes living along the course of the Thames, whose names are given by Cæsar as Cenimagni, Segontiaci, Ancalites, Bibroci, and Cassi, all of whom submitted. From them Cæsar learnt that Cassivellaunus had not far distant a fortified place in which a large number both of men and of cattle had been collected for protection against the enemy; this stronghold was promptly attacked by Cæsar; its defenders were unable to repulse Cæsar's attack and made their escape on another side. Many of them were killed in their flight, and the whole of the cattle fell into Cæsar's hands. The precise position of this place is unknown. Meanwhile Cassivellaunus sent instructions to the four kings who governed as many districts in Cantium, or Kent, to surprise and storm Cæsar's naval camp. The attempt failed, and, being discouraged by his own ill-success, and still more by the defection of his allies, Cassivellaunus submitted to Cæsar, who took hostages, imposed an annual tribute, and enjoined Cassivellaunus to abstain from harassing the Trinovantes or their king Mandubratius. Cæsar now left Britain, after a stay of barely two months. In Welsh tradition, as preserved in the Triads and the Bruts, Cassivellaunus appears as Caswallawn. Here much romantic detail overlies a narrative in which an agreement with the main outline of Cæsar's account can be traced.
The name Cassivellaunus is Gaulish in form. The first part of the word is compared by Professor Rhys with the name of the tribe of the Cassi, and the whole is interpreted by him to mean ‘a ruler of the league or a tribe-king.’ Vellaunus probably meant ‘a ruler,’ being connected with the Irish flaith (a prince), and with Welsh gwlad (country), English wield. The name of the Catuvellauni is similarly compounded of vellauni with catu, Irish cath, Welsh cad, battle.
[Cæsar, B. G. v. 11–23; Elton's Origins of English History; Rhys's Lectures on Welsh Philology, 2nd ed., and Celtic Britain.]