King Arthur is now thought to have been a real person, a Celtic ruler in Britain about the time of the last Romans in Britain, say during the rule of Claudius as emperor. In the stories as preserved, however, he was shown as a medieval king of a disunited Britain who was trying to bring the country together. The purpose of the table being round was that in those days rank was shown by whoever sat closest to the king and the foot of the table was lowest in rank. By making the table round, this system was thwarted, making all equal in rank, all equidistant from one another. Merlin was the sorcerer in the medieval tales, but he was the religious adviser in the original tales, which were pre-Christian (so he might have been an ancient priest of the Druidic type, or a shaman). Some of the Arthurian tales have interesting parallels in the lore of some people of the Caucasus mountains, the Ossetes, as shown in an article in the "Journal of American Folklore" (I think this was in the 1970s but you'd have to check this online). The author proposes that some warriors from the Caucausus were posted to Britain by the Romans and they provided the source of these legends. This was the basis of the recent movie in which the warriors are Sarmatians. However, it is far more likely that these elements are much older and come from the Proto-Indo-European culture which gave rise to the Celts, the Sarmatians, and to a great many other cultures between these two as well, all of which you can look up on Google. Some of the famous characters in Arthurian legend include Bedevere, Guinevere (Arthur's queen), Sir Lancelot (who loved Guinevere), and the virtuous Parsival who even had an opera written about him. Geoffrey of Monmouth wrote an early version of the Arthurian tales in his history, called "The History of the Kings of Britain." One of the most famous versions is the more modern "The Once and Future King" by T.H. White. It has this name because at his death Arthur was thought to have been carried off to the Blessed Isles in the west where he was only waiting for Britain to need him, when he would return, Christ-like, to rescue his homeland again.
MS sociology, minor in anthropology with emphasis on folklore; always a fan of Arthurian legend -- start at Wikipedia but check each of the footnotes as they tell the REAL stories