This one falls loosely into the "fantasy" category. It is a very different take on the Arthurian legend, and is claimed to be far more realistic than the usual medieval mythology. The hero, Artorius Castus (Clive Owen), is depicted as a commander of the Sarmatian cavalry, an auxiliary force to the Roman Army, based in Britain by Hadrian’s Wall at the time of the Roman departure in the fifth century AD. He and his men have to cope with a Saxon invasion as well as incursions by the savage "Woads" (Picts, who allegedly adorned themselves with woad – blue dye – when preparing for battle).
Artorius's "knights" are a rough and shaggy lot of pagans (light years from the usual virginal Christian knights in shining armour) who have earned their impressive reputation the hard way, in savage hand-to-hand fighting. There's quite a lot of that in the film. They have come to the end of their fifteen-year contracts of service with Rome and are looking forward to returning home to the Middle East when they are ordered on one last, seemingly suicidal mission, over the wall and into Woad territory, right in the path of the invading Saxons. The end result is the climactic battle of Mount Badon.
About the only relationship to the traditional Arthurian characters is the names: not just Artorious/Arthur but Lancelot, Galahad, Bors, Tristan and Gawain all feature. Merlin is the leader of the Woads (with no suggestion of magical powers) and there's also Guinevere (Keira Knightley), a feisty Woad warrior whom the Sarmatians pick up on the way. A round table is slipped in (in a different context) but there's no mention of Camelot or the Holy Grail.
Is it realistic? Well, the setting is historically not far out; the first and only credible mention of Arthur (written centuries after he is believed to have lived) places him in approximately that time and place, although the timing is compressed somewhat - the Romans were long gone by the the late fifth/early sixth century, when he is historically supposed to have lived. The Battle of Mount Badon is believed to have some basis in fact, although no-one knows where it actually happened. And in the first tales he was no king, but a noted warrior who fought alongside the British kings. All of the medieval twaddle which has since accrued around the Arthurian myth was entirely invented from the twelfth century onwards.
Clive Owen makes a good fist of the Arthur role, and on the whole I enjoyed the film. For me, the least realistic element was Guinevere - or rather, Keira Knightley. She is a photogenic and popular young actress who is often cast for that reason - and quite frequently miscast, as in this film. My suspension of disbelief slipped badly the moment I heard her cut-glass voice emerged from a supposedly savage Woad. While she does a reasonably good bloodthirsty impression, I couldn't see the skinny arms of her size zero body pulling a war bow or wielding a sword in battle. And where did she get lipstick from? Oh well, I suppose that some cinematic conventions must be determinedly protected against the onset of too much reality.