In the wooded terrain north of the River Thames, Cassivellaunus adopted scorched-earth, guerrilla-warfare methods, destroying local food sources and using chariots to harrass the Roman legions. But neighboring tribes who resented the domination by Cassivellaunus, including the Trinovantes and their allies the Cenimagni, Segontiaci, Ancalites, Bibroci and Cassi (the latter five tribes, known to us only through Caesar's account) then went over to the Romans.
Caesar thus learned from native informants the location of the secret stronghold of Cassivellaunus, probably the hill fort at Wheathampstead, located on the west bank of the River Lea, near St. Albans. Even as the Roman army under Caesar were massing outside his fort's gates, however, Cassivellaunus made the bold move of ordering his allies in Kent to attack the Roman beach camp at Deal. This attack failed, and Cassivellaunus then gave up. Yet the terms of surrender he negotiated with the Romans seem to have been moderate, as Caesar had learned of mounting problems back in Gaul, and wanted to return there. The Roman legions left Britain in early September, 54 BC. They were not to return again for 97 years, when the Claudian invasion of AD 43 began the active Roman conquest of Britain. Caesar's two expeditions, meanwhile, provided basic information on the terrain, inhabitants, and political, economic and military customs of Britain, our only direct historical record for that time period.