The Celts arrived in Britain and their religious leaders were called Druids. They worshipped gods and goddesses and performed sacrifices. Britain was later conquered by the Romans who brought Christianity. Arthurian legends first appeared in writings by Geoffrey of Monmouth in the 12th century, though Arthur may have been based on a real 5th century British king named Riothamus. Prior to Arthur's birth, his ancestors included a Trojan prince who fled to Britain and Constantine whose son Uther Pendragon was Arthur's father. Uther was told in a dream his son Arthur would rule Britain.