Just a couple of points there. Arthur was mentioned in early Welsh literature - as was Merlin, but never in the same stories. In these stories, Arthur was never called a king and the details are pretty sketchy - there's more known about Merlin.
Geoffrey of Monmouth was a Norman who was basically rewriting British history and trying to create a unity between the Britons, Saxons and the Normans. The irony is that the Saxons that Arthur beat were the English. Kind of surprising that the English embraced Arthur as their hero.
Most of the Arthur myth was written by the French and such things as Camelot and Lancelot were added then.