The Catuvellauni emerged in the late first century BC to become one of the most powerful tribes in southern Britain. They were bordered to the north by the Corieltavi, to the east by the Iceni and Trinovantes, to the south by the Atrebates, and to the west by the Dobunni and Cornovii. Like many of their neighbours in the south-east, they were probably a Belgic tribe from the North Sea or Baltics. They may have been related to the Catalauni, a Belgic tribe of Gaul.
The main territory of the Catuvellauni lay on the northern bank of the River Thames, and northwards from there (in modern Hertfordshire). This is the area of their original powerbase, and also where Julius Caesar places a tribe he named as the Cassi in 54 BC. The tribe's early capital was at Wheathampstead, and under Cassivellaunus they expanded outwards to dominate Cambridgeshire, Northamptonshire, Bedfordshire, Buckinghamshire, Oxfordshire east of the Cherwell, Middlesex and north-east Surrey.
They were one of the most prominent Celtic tribes of their time, and also one of the richest. They were good agriculturalists and had some of the best soil in the country on which to farm. Nevertheless, as with all the pre-Roman Celts, they left no written records. Their rulers are only noted after they began issuing coinage or came into contact with the Romans. (The History Files).
BNJ Refs: The Bronze Coinage of Tasciovanos, BNJ 85 (2015), Rainer Kretz