In Britain, they weren't.
In June 1792, a group of bards in London staged an entirely fictitious ceremony on Primrose Hill involving a stone circle made from pebbles and claimed they were reviving an ancient ritual that stretched right back 'to the ancient Celtic nation and it's druids'.
Prior to this there is no record of the word 'Celts' being used to describe the pre-Roman inhabitants of Britain or Ireland, and it was certainly not a term they used themselves.
The word 'Celt' was coined by the Greek historian Herodotus in 450 BC to describe the people living near the Northern side of the Danube north of the alps.
The Roman name for such people were 'Galli' - 'chicken people' - and they called the inhabitants of the British Isles as 'Britanni', never Celts.
A Welsh linguist living in Oxford in the 17th Century noted the similarities between the languages spoken in Ireland, Wales, Scotland, Cornwall and Brittany. He called these languages 'Celtic' and the name stuck, even though there is no such thing as an ethnically homogenous Celtic nation.
The romanticised idea of a Celtic Empire of horse-loving master craftsmen, wise old druids, harp-strumming poets and fierce bearded warriors is the product of the Celtic Revival that started in the late 18th Century, which has more to do with modern Irish, Welsh and Scottish nationalism than with any historical reality.
The celts did not have pencils.
did the celts have any illnesses
Yes, the Celts practiced various burial customs, including burying their dead in simple pits or elaborate tombs. Some Celts were also cremated, with their ashes placed in urns or buried in graves. Each Celtic community had its own burial traditions and practices.
The leader of the Celts was queen boudicca
When did Celts come to Britain
The picts (the painted people) were celts.
The Celts were in Europe.
Celts have not died out. Descendants of the Celts can be found living in Cornwall, Scotland, Ireland, Wales and Brittany.
Yes the Celts do speak English
Cincinnati Celts was created in 1910.
Orthodox Celts was created in 1992.
They were there to protect the Celts ! and use for food