The King Arthur tales took their form in the 15th century as the pinnacle of medieval ideals. But the ancient sources point to a historical Arthur, dating 1,000 years earlier in a very different stage of civilization. The Dark Ages (not to be confused with the Middle Ages) were the centuries after the fall of Rome.
Britain was a Celtic land that had been conquered by the Romans. When Rome became Christian, so did Britain, except for the wild tribes north of Hadrian's wall. But when the Empire declined, the legions pulled out and left the Romanized Celts all but helpless against a brutal Saxon invasion. According to the old chronicles, a leader with the Roman name Artorius defeated the Saxons at the Battle of Mt. Badon. This was Arthur, who became a legend.
The new movie King Arthur (rated PG-13) is about the historical Artorius, filling in substantial gaps in the record with plausible history, fictional speculation, and a few distractingly modern preoccupations.
This Arthur is a Roman officer who stays behind when the Romans leave and leads an elite troop of cavalry. Their names are Lancelot, Gawaine, Tristan, Bors, etc. (But the knights have nothing to do with their legends, and all of them are coarse and bawdy, which, along with a premarital sexual encounter, makes this not a movie for children.) We see Arthur nudging his men forward into what would become chivalry as they battle wild, painted, un-Romanized Celts called Woads and led by a spooky fellow called Merlin.
It is fun to pick up on realistic details presented as the origins of the legends. (For example, the young Arthur pulls his sword not out of a stone but his father's grave.) But then there are the contemporary touches. Guinevere is a Celtic, proto-feminist warrior maiden. Arthur is a follower of Pelagius, the British monk who insisted that we are saved by our own free will and not the grace of God. The movie takes this denial of the gospel as an assertion of human freedom and equality in today's political sense! Orthodox Christians are oily and corrupt and, as is the vogue in contemporary liberal scholarship, the heretics are the good guys.
In many ways, King Arthur is a good movie, with strong characters, a good story, and terrific battle scenes. It is just not quite ancient enough.