Cassivellaunus was a historical British tribal chief who led the defence against Julius Caesar's second expedition to Britain in 54 BC. He led an alliance of tribes against Roman forces, but eventually surrendered after his location was revealed to Julius Caesar by defeated Britons.
A plaque in Devil's Dyke mentions Cassivellaunus
Cassivellaunus was given command of the combined British forces opposing Caesar's second invasion of Britain. Caesar tells us that Cassivellaunus had previously been in near-constant conflict with his neighbors, as was typical of the British tribes in this period, and had recently brought down 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 possibly near the Devil's Dyke in Sussex.
Caesar proceeded to put the stronghold 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. The terms were mediated by Commius, Caesar's Gallic ally. 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  where a poor harvest had caused unrest. The Roman legions did not return to Britain for another 97 years.
Cassibelaunus the younger son of the former king Heli, he becomes king of Britain upon the death of his elder brother Lud, whose own sons Androgeus and Tenvantius are not yet of age. In recompense, Androgeus is made Duke of Kent and Trinovantum (London), and Tenvantius is made Duke of Cornwall.
After his conquest of Gaul, Julius Caesar sets his sights on Britain, and sends a letter to Cassibelanus demanding tribute. Cassibelanus refuses, citing the Britons' and Romans' common Trojan descent and Caesar invades at the Thames Estuary. During the fighting, Cassibelanus's brother Nennius encounters Caesar and sustains a severe head wound. Caesar's sword gets stuck in Nennius's shield, and when the two are separated in the mêlée, Nennius throws away his own sword and attacks the Romans with Caesar's, killing many, including the tribune Labienus. The Britons hold firm, and that night Caesar flees back to Gaul. Cassibelanus's celebrations are muted by Nennius's death from his head wound. He is buried with the sword he took from Caesar, which is named Yellow Death..
Two years later, Caesar invades again with a larger force. Cassibelanus, forewarned, had planted stakes beneath the waterline of the Thames which gut Caesar's ships, drowning thousands of men. The Romans are once again quickly put to flight.
The leaders of the Britons gather in Trinovantum to thank the gods for their victory with many animal sacrifices and celebrate with sporting events. During a wrestling bout, Cassibelanus's nephew Hirelglas is killed by Androgeus's nephew Cuelinus. Cassibelanus demands that Androgeus turn his nephew over to him for trial, but Androgeus refuses, insisting he should be tried in his own court in Trinovantum. Cassibelanus threatens war, and Androgeus appeals to Caesar for help, agreeing to accept him as liege and sending his son as a hostage.
Caesar invades a third time, landing at Richborough. As Cassibelaunus's army meets Caesar's, Androgeus attacks Cassibelaunus from the rear with five thousand men. His line broken, Cassibelanus retreats to a nearby hilltop. After two days siege, Androgeus appeals to Caesar to offer terms. Cassibelanus agrees to pay tribute of three thousand pounds of silver, and he and Caesar become friends.
Six years later, Cassibelanus dies and is buried in York. Androgeus has gone to Rome with Caesar, so Tenvantius succeeds as king of Britain.