Caesar tells us that Cassivellaunus had previously been at constant war with the British tribes, and had overthrown the king of the Trinovantes, the most powerful tribe in Britain at the time. The king's son, Mandubracius, fled to Caesar in Gaul.
Despite Cassivellaunus's harrying tactics, designed to prevent Caesar's army from foraging and plundering for food, Caesar advanced to the Thames. The only fordable point was defended and fortified with sharp stakes, but the Romans managed to cross it. Cassivellaunus dismissed most of his army and resorted to guerilla tactics, relying on his knowledge of the territory and the speed of his chariots.
Five British tribes, the Cenimagni, the Segontiaci, the Ancalites, the Bibroci and the Cassi, surrendered to Caesar and revealed the location of Cassivellaunus's stronghold, which Caesar proceeded to put under siege. Cassivellaunus managed to get a message to the four kings of Kent, Cingetorix, Carvilius, Taximagulus and Segovax, to gather their forces and attack the Roman camp on the coast, but the Romans defended themselves successfully, capturing a chieftain called Lugotorix. On hearing of the defeat and the devastation of his territories, Cassivellaunus surrendered. Hostages were given and a tribute agreed. Mandubracius was restored to the kingship of the Trinovantes, and Cassivellaunus undertook not to wage war against him. All this achieved, Caesar returned to Gaul.
| Preceded by: -- | King of the Catuvellauni | Succeeded by:<br.>Tasciovanus |
Soon after Lud's death, Cassibelanus was crowned king in favor of his two young nephews whom he had been serving for as regent. He gave to his elder nephew the duchy of Kent and Trinovantum (London), and gave to his younger nephew, Tenvantius, the duchy of Cornwall.
Sometime in the beginning of his reign, Julius Caesar requested the British to pay tribute to Rome as a means of avoiding warfare between the two kingdoms which claim descent from the Trojans. Cassibelanus refused the offer and Caesar invaded the country along the banks of the River Thames. After much fighting, Nennius, Cassibelanus's brother, stole Caesar's sword and led the army of the Britons successfully against the Romans. They pushed the Romans back to their ships where they fled to Gaul and brooded over their loss.
The Romans waited two years until they returned to Britain. Cassibelanus rebuilt city walls and defences throughout Britain while awaiting Caesar's attack. Most famously, he commanded that large spikes be placed in the water beneith the Thames so that the bottom of the Roman ships would be gutted as they floated down the river. Mass conscription went out among the Britons and barracks were constructed all along the river. When the Romans invaded again they were devastated in the river and along the shores of the River Thames. After numerous charges, the Romans were defeated and forced out of Britain once again.
Cassibelanus celebrated the victory with massive sacrificing of animals in London. It was at this celebration that Androgeus and Cassibelanus fought over the death of a relative. Cassibelanus became angry and ravaged the lands of Androgeus causing Androgeus to seek the aid of Caesar. Caesar invaded for a third time and occupied London due to the help of Androgeus. Cassibelanus counterattacked but was not able to defeat Caesar so he fled to a hill outside the city and held his ground. All night Caesar attacked them then he continued a siege of the hill for two days, starving out the Britons.
Fearing doom, Cassibelanus wrote to Androgeus seeking peace with him and Caesar. Androgeus discussed terms with Caesar and Cassibelanus agreed to pay tribute annually to Rome. Caesar and Cassibelanus became friends over that winter while Caesar remained in Britain, then Caesar returned to Gaul and did not return.
Cassibelanus died six years after that winter and was buried in York. His nephew, Tenvantius, succeeded him to the kingship.
In the Mabinogion, Caswallawn son of Beli is mentioned as having seized the throne of Britain from Bran the Blessed while the latter was fighting a war in Ireland. In the Welsh Triads Caswallawn is said to have crossed the sea with 21,000 men in pursuit of Caesar, and never returned. His horse is named as Meinlas and his lover as Fflur, daughter of Mygnach the Dwarf. According to Iolo Morgannwg's collection of triads, Caswallawn had rescued the abducted Fflur from Caesar in Gaul, killing 6,000 Romans, and Caesar invaded Britain in response.
| Preceded by: Androgeus Regent: Cassibelanus | Mythical British Kings | Succeeded by:<br.>Tenvantius |