In OTL "King Arthur" is a semi-mythical figure who is generally accepted as having a historical basis, whether a single person or a fusion of several historical characters. One possibility is that Arthur was a Roman centurion or standard bearer (the name has a connection with a word for 'bear' and standard bearers wore bearskins as part of their uniform), and his 'knights' were the (ex)legionnaries associated with him. In this timeline Arthur becomes a more recognisably historical figure, being the founder of a successor state to the Romans, based on Glastonbury.