The Druids were the religious and legal gurus of ancient Celtic society, which existed all over Europe and even parts of Asia up till Roman times. Following the Roman conquest of Britain from about 43 AD onwards, the British Celts retreated to Scotland, Wales and Ireland. Hard and fast information about the Druids is hard to come by as they seem to have been a deliberately secretive and mysterious priesthood but Julius Caesar provides some of the earliest written evidence about them in his account of his own two aborted invasions of Britain in the 50s BC. He describes them as exercising great power over the tribes and even over their leaders. Caesar also mentions human sacrifice as part of their rituals and, although this has been dismissed as Roman propaganda in the past, more recent research suggests that such sacrifices did indeed take place and in fact became more and more frequent as Celtic resistance to the Roman invaders became increasingly more desperate.