When did the Celts reign?

The Celts were a diverse group of tribal societies that flourished in Europe from around 800 BC to the Roman conquest in the 1st century AD. They reached their peak during the Iron Age, particularly between 500 BC and 1 AD, when they spread across large parts of Western and Central Europe. Their influence began to wane as the Roman Empire expanded and assimilated many Celtic territories. By the early Middle Ages, the Celts had largely been absorbed into the cultures of the regions they inhabited.