The Celts arrived in Britain during a large migration of people from Europe westwards during the early Iron Age. A nation of fierce warriors, the tribes of the Brythonic Celts inhabited England, Wales and lowland Scotland during the Roman era and the post-Roman era. Their Religious practices revolved around offerings and sacrifices, sometimes human but more often involving the ritual slaughter of animals or the deposition of metalwork, especially war booty.
The Celts of England spoke a language known as Brythonic Celtic which developed from Proto-Celtic, which was to evolve into modern Welsh in Wales and Cumbric in the Hen Ogledd or "Old North" of Britain, Cornish in Cornwall and Breton in Gaul.
The Roman Emperor Claudius invaded Britain in 43 AD and encountered resistance from the Celts under the leadership of Caractacus, king of the Catuvellauni tribe and later by Boudica, Queen of the Icceni of Norfolk.
On the departure of the Roman legions and the arrival of the Anglo-Saxons under the brothers Hengist and Horsa in the fifth century, many of the Brythonic Celtic people were either absorbed into Anglo-Saxon culture, becoming "English"; some retreated to the Celtic enclaves of Wales, Cornwall and southern Scotland, while some emigrated to Brittany. Celtic resistance to the invaders had collapsed by 580, the Welsh monk Gildas relates "the groans of the Britons" , "The barbarians" he states, "drove them to the sea and the sea drove them back to the barbarians."