In this reputedly "more historical" version of the fabled King Arthur's story, don't expect to see any sorcerers or magical swords commonly associated with the medieval Arthurian legends. King Arthur is set in the year 475 or so, and it's a messy, bleak time.
The Romans are leaving Britain, and they dispatch their top knight, Arthur, and his band of highly trained Sarmatian knights (think Special Forces assigned to dirty work) on one last mission. This quest ultimately shatters any remaining illusions Arthur holds about his future with Rome, and under the influence of the indigenous people, the Woads (including the comely Guinevere), he decides to do a little nation-building.
When the Romans split, there's a power vacuum between the invading fur-draped Saxons under the command of Cerdic (Stellan Skarsgí¥rd) and the Woads (i.e. manic, blue-painted tree-people) led by a non-magical Merlin. Though this film features three groups of savage, slash-first-ask-questions-later warriors, it's Arthur's cause that is noble: He wants to make Britain a free land, for free men -- and he'll kill thousands to make it so.
So what of the battles? They are noisy, but inexpertly staged. If slow motion is the new emotional resonance, then sound effects are the new gore. There are resounding clangs, a lot of battery (but very little blood) and in the final climatic battle, big fiery explosions, but the warfare scenes are mostly a melee of swinging arms and rearing horses. And one rather ingenious set-up -- a mid-film battle on ice -- is ultimately marred by special effects better suited to an outrageous disaster flick.
Clearly, director Antoine Fuqua (Training Day) is aiming for a gritty naturalism, but King Arthur also features scenes that are reminiscent of the cheesiness of mid-century Hollywood spectacles -- perfect beams of light through the trees, a lost little orphan boy, and Guinevere walking through the snow in a divine, off-the-shoulder silken gown.
As the half-British, half-Roman Arthur, Owen is moody throughout, though the charitable might say "preoccupied," as much of Western Civilization would appear to rest on his chain-mailed shoulders. As fair Guinevere, Keira Knightley is maturing into a very attractive woman, though her plummy tones and flawless complexion seem inconsistent with a wild tree-dweller.
And while it might be historically accurate that such women fought alongside men back in the Dark Ages, Guinevere ill-advisedly goes into battle wearing what amounts to a thin leather strap across her nipples and some tattoos. As such, Fuqua cuts frequently to Knightley, as her nubile, near-naked torso twists and turns, and her long locks curled just-so swing madly. These rather gratuitous shots should have just been marked: "Teen-age fanboy, download here." 
