Short answers, pulled from the story.
The first recorded use of the term Celtos appeared in 517 BC by the Greek geographer Hecataeus of Miletus. This historical record described a people living near Massilia, which is modern-day Marseille.
The etymology of the word Celt remains a subject of intense debate with possible roots meaning to hide, to heat, or to impel. These definitions suggest a people who were once hidden from history or perhaps those who drove forward with great force.
The mainstream theory links the Celts to the Urnfield culture of central Europe around 1000 BC and the subsequent Hallstatt culture which developed from 800 BC to 500 BC. The La Tène culture emerged around 450 BC and became the archaeological expression of the Celts by the 1870s.
The Celts expanded down the Danube river and its tributaries to establish a presence stretching from the British Isles to central Anatolia. One of the most influential tribes, the Scordisci, established their capital at Singidunum, present-day Belgrade, Serbia, in the 3rd century BC.
The principal Celtic weapon was a long bladed sword which was used for hacking edgewise rather than stabbing. In addition to these swords, spears and specialized javelins were also used, and some Celts fought naked to present a terrifying spectacle.
By c. 500, due to Romanisation and the migration of Germanic tribes, Celtic culture had mostly become restricted to Ireland, western and northern Britain, and Brittany. Between the 5th and 8th centuries, the Celtic-speaking communities in these Atlantic regions emerged as a reasonably cohesive cultural entity.