With the expansion of the Roman world into western Europe, written histories and geographies record the customs and organisation of the native peoples'. Many of our ideas about the Celts derive from Julius Caesar's accounts of his conquest of Gaul.

The classical writers never referred to the peoples of Britain as either Celts or Gauls, instead they were named in relation to tribes, leaders and the geography of the islands. What we know as Wales today was, according to Roman sources, inhabited by at least four tribal peoples by the time of the invasion (AD43-70): the Ordovices (north-west), the Deceangli (north-east), the Demetae (south-west) and the Silures (south-east).