Cassivellaunus is probably Wheathampstead's most famous "resident". He was leader of the Catuvellauni tribe that ruled this area at the time of the first Roman invasion in 54 BC. He was a powerful chieftain who subjugated many of the other local tribes including the powerful Trinovantes in what is now southern East Anglia. When Caesar invaded, Cassivellaunus led the defence.

Devil's Dyke was a significant Iron Age site and thought to be the main settlement of the Catuvellauni.  Some historians have claimed that Julius Caesar attacked and defeated Cassivellaunus here but this theory is now discredited.

Julius Caesar wrote an account of his invasion and Cassivellaunus became the first named Briton ever to be recorded in history.