Who is Cassivellaunus from The Conquest of Gaul and what is their importance?

Cassivellaunus is the chief of the British. The men under his command resist Caesar's second invasion. Cassivellaunus' charioteers conduct a campaign of harassment against the Roman column as it moves through their territory. This tactic successfully confines Caesar's activities. When Caesar later attacks Cassivellaunus he is successful. After Cassivellaunus's surrender, Caesar returns to Gaul.