At the turn of the first millennium BC, there was a proto-Celt society in Europe now known as the "Urnfield People." These bronze age warriors had spread from the Scythian mountains in the east, to eastern France by c. 800 BC and can truly be described as the forefathers of what is now called the Celts.
Following on from the Urnfield people, came the Hallstatt culture in around 900 BC to 500. In about 700 BC, these people were true Celts as we understand it today, and brought with them the massive advantage of Iron. The next three hundred years became known as the early Iron Age and can probably be described as the "Celtic Era."
The La Tene culture lasted from 500 BC to 15 BC. This was the central era for celtic expansion and migration and the time when the celtic myths were formed.
The Celts settled in Spain in 450 BC, in Northern Italy in 400 BC, invaded Rome in 390 BC, invaded Greece in 279 BC, invaded Galatia in 270 BC, and by 200 BC, they occupied the lands that are now Britain, the Netherlands, Brittany, Belgium, Germany and Switzerland.
However, in 82 BC, the Romans defeated the Celts in Italy. In 55 BC Caesar defeated the Celts in Gaul.
The years from 55 BC to 409 AD were the years of Roman rule in Britain. Although the Romans ruled through accomodation with the Celtic tribes, carrying out a policy of Romanising the local leaders, they eradicated any Celtic opposition. They attacked the Druidic centre in Anglesey in 61AD.