Cassivellaunus was a historical British military leader 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.

He appears in British legend as Cassibelanus, one of Geoffrey of Monmouth's kings of the Britons, and in the Mabinogi, Brut y Brenhinedd and the Welsh Triads as Caswallawn, son of Beli Mawr.

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.

The Greek author Polyaenus relates an anecdote in his Stratagemata that Caesar overcame Cassivellaunus's defence of a river crossing by means of an armoured elephant.

[6] Cassivellaunus appears in Geoffrey of Monmouth's 12th century work Historia Regum Britanniae (History of the Kings of Britain), usually spelled Cassibelanus or Cassibelaunus.

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 (see Brutus of Britain), and Caesar invades at the Thames Estuary.

Cassibelanus, forewarned, had planted stakes beneath the waterline of the Thames which gut Caesar's ships, drowning thousands of men.

Cassibelanus threatens war, and Androgeus appeals to Caesar for help, agreeing to accept him as liege and sending his son as a hostage.

In the Second Branch of the Mabinogi, he appears as a usurper, who seizes the throne of Britain while the rightful king, Bran the Blessed, is at war in Ireland.

However, the 12th-century poet Cynddelw Brydydd Mawr knew of some version of the Fflur story, writing that Caesar's love for her was costly.

[20] Welsh scholar Rachel Bromwich suggests the fragmentary allusions to Caswallawn in the Triads relate to a narrative of the character that has been lost.