Cassivellaunus (kă'sĭvĭlô`nəs), fl. 54 B.C., British chieftain, a leader in the resistance against the invasion of Julius Caesar in 54 B.C. Caesar crossed the Thames River into Cassivellaunus's home country. Aided by discontented British tribes, he attacked Cassivellaunus in his strong fort in the marshes (probably at Wheathampstead, Hertfordshire) and drove the Britons out with heavy losses. Cassivellaunus sued for peace, which Caesar granted in return for hostages and an annual tribute.