Ken Hite mercilessly summarizes the latest film’s take on king Arthur:
Its "Arthur" is the (imaginary) lineal descendant of Lucius Artorius Castus (fl. ca. 185 A.D.), himself the only Roman official in Britain known to have borne the name Artorius.
Its "knights" are Sarmatian cataphracti, an interesting (if ridiculously presented) take on a theory proposed first (AFAIK) in the 1920s, and elaborated well beyond all sanity in the terrifically interesting From Scythia to Camelot by Littleton and Malcor.
Its "Britain" is the Land of Silly Stupid Pretend.
For further reading, he recommends Tom Green’s Arthurian Resources (or N.J. Higham’s King Arthur: Myth-Making and History, but warns:
Do not come to either Green or Higham with a romantic spirit, however, or they shall crush you like Arthur may or may not have crushed the Saxons, possibly at Badon Hill, wherever that was, around 540 A.D. or thereabouts, if he existed at all, which there’s barely any scholarly reason to say that he did.
There. I’ve copy-pasted practically all of his entry, except for the bit about The number of Keira Knightley square-inch-seconds
being worth the price of the ticket.
Oops.