The Celts are a playable civilization.
The Celts were a group of Indo-Europeans who inhabited much of Central and Western Europe in the period between 1000 and 500 BC. While Celtic culture today is associated with Ireland, Scotland and Wales, the ancient Celts were spread out over a much larger distance. In addition to those locations, different groups of Celts were also present in Spain, France (Gaul), Hungary, and Bulgaria (Thrace). Although the Greeks and Romans considered the Celts to be barbarians, they possessed a sophisticated and unique culture.
The ancient Celts of Gaul and Britain (who we know the most about) were divided into many tribes loosely based on kinship ties. Social structure was determined by a mixture of class standing within the tribe and one's place within the Celtic religion. Kings were elected to lead the tribes, and society was divided into the three groups: warriors, an intellectual class of druids and bards, and everyone else. Society was based around warfare, but to the Celts it was more of a sport than a battle to the death: raids were the norm, and pitched battles were rare.
The Celts do not appear to have had any large cities, although there were many small towns and fortresses across the regions they controlled. Trade was rather primitive and mostly consisted of a barter economy, although in some coastal regions it was more developed. The Celts shared a common religion emphasizing sacred groves, the role of druids as priests and scholars, and the use of periodic sacrifices (which were occasionally human sacrifices). Celtic gods were named after natural phenomenon and they were quite numerous. Other Celtic practices included the removal of body hair (done to ensure cleanliness) and the taking of enemy heads as trophies in battle.
The most notable event in the early history of the Celts was the sacking of the city of Rome in 390 BC under the great King Brennus. Although the victory was a military triumph and provided a large immediate gain to the Celts, the Romans would prove to have a very long memory, and over the years they would make their enemies pay dearly for it by methodically conquering all Celtic territory their mighty legions could reach.
By 192 BC, the Romans had subjugated the last of the Celtic tribes in Italy, and they then began a program of colonization of southern Gaul on the Mediterranean coast. Under the great general Julius Caesar, the Romans conquered the entirety of Gaul and defeated a large revolt headed by Vercingetorix between 58 - 51 BC, removing the Celtic threat to the Imperial City once and for all. Under the Emperor Claudius, the Romans went on to invade and occupy Celtic Britain as well.
The Celts remained under Roman rule for the next four centuries until the Germanic invasions of the fifth century AD destroyed the Roman Empire in the west. Although Celtic culture disappeared in many parts of Europe during those years of foreign occupation, it was preserved in Ireland and the western fringe of Britain. Over time the pagan Celtic culture would combine with Christianity to give rise to beautiful new forms of artwork and literature. Despite many attempts over the centuries to destroy it, the Celtic culture survives. In fact, today it enjoys a great deal of popularity and is even experiencing something of a revival.