Cassivellaunus was a historical British chieftain who led the defence against Julius Caesar's second expedition to Britain in 54 BC. He also appears in British legend as Cassibelanus, one of Geoffrey of Monmouth's kings of Britain, and in the Mabinogion and Welsh Triads as Caswallawn, son of Beli Mawr. History Cassivellaunus is the first British individual known to history. He appears in Julius Caesar's Commentaries on the Gallic War, having been given command of the combined British forces oppo… (More on Cassivellaunus)