You hit my favorite topic. I don't think there was a horse riding, banners-waving King Arthur who lived in a stone castle. That type of story was invented by the Old French. But I do think there may have been a Celtic warlord with the title Artos ("the bear" in Celtic). "Camelot" could be a corruption of "Caer Mallot" or castle of the king, in Celtic (the mallot, or hammer, was the weapon traditionally associated with the king). Also, there may have truly been a Lancelot. The Old French could have been L'Ancelot, or the Ancelot. The translation from Old French to Latin would have been Anguselsus, or the Celtic Angus. If he were THE Angus, or L'Ancelot, it would have meant he was the leader of his clan.
The Old French also looked down on the Celts, so whenever possible they translated words into slightly different meanings. The Celtic word for "table," as in "round table," also translates into "building," and there were many round buildings, so Arthur could have met with his warriors in a round building. Also, the French stories said Guinevere got up from a bloody "bed" together. The word also means "altar," and the Picts of England had warrier priestesses, so she could have have been that, and Lancelot could have been a fellow worshipper at a bloody altar (yes, they did that kind of thing back then). In a Christian court, this would have been pretty shocking, but not as bad as if Guinevere really were cheating on Arthur.
There is a series of scholarly books out tracing the possible origins of Arthur, Merlin, Guinevere, and Lancelot