To start with - King Arthur: Legend of the Sword isn't strictly speaking a period drama, but neither is The White Queen with its britanized witchy voodoo or Reign with its mysticism and time-travelling costumes. However, Arthurian legends are far too important to ignore when it comes to historical fiction, whether we like it or not.
First impression
When I saw the trailer, I hated it. I didn't need to see the film to assume it ruined everything I hold dearly about Medieval England - I mean, the amount of modern montage and special effects just exploded into my face, as if the film aimed to be like Avatar or Harry Potter - full of magic... that's supposed to be believable?
Opening sequence
And, in all fairness, the opening sequence did not only annoy me with the excessive over the top actionness and giant elephants - it also either referenced or bluntly ripped off The Lord of the Rings bridge fighting scene. Whichever it is, the exposition battle did not sell it to me. More than that, it made me want to watch the film even less
As it turns out, don't judge a film by the first 10-15 minutes
It did get better. Although entirely not what you'd want from the legend of king Arthur, the film is really well-made - even the overly modern ageing montage works. It is nice to see a hero that's competent and strong in his own right already - and actually is prepared for his destiny somewhat, though thanks to his unlikely upbringing in a brother rather than fate.
Arthur
I'm in two minds about Arthur. His modern body language does irritate me - simply because I can't imagine that it was exactly like that back then, especially in Medieval Britain. Ritchie's Arthur strikes me as too much of an American street boy than a British underdog who takes what he actually deserves.
He is a charmer in some way though. And placing him in a brothel does provide a valid explanation for why he is strong-willed, determined, physically strong and would keep his friends close in the end. This Arthur is believable enough due to that ageing montage I mentioned earlier. And he does add the sense of light-hearted humour that period drama lacks sometimes.
Modernization
Now... it's a tough one. It does work - but just about. I'm not even a history buff but I read Le Morte d'Arthur at least partly, have seen the Clive Owen take on the characters and have seen enough Medieval films to be sceptical about 'unusual' takes on the classic. I also watched BBC Merlin series when I was younger (and I'm convinced Katie McGrath's presence in this film is a nod to the childish magic series...), so it's not the first modern(ish) Arthur on screen I'm familiar with. However, Merlin only took the modern friendship dynamic and built a story on it, while in King Arthur the 'mates' dynamic isn't as charming and stands out a bit too much. It really reminded me of Marvel's Antman at times, actually.
Overall
The thing is, the film is well-made and enjoyable. The acting is on point - even if for some people it's not what they'd want from an Arthurian legend. Some of the things, - like the giant snake... and fighting giant rats - I mean, surely a tough guy shouldn't be scared of that? I expected like a sentimental puzzle that will leave Arthur crying like a baby after the witch's speech, but not a couple of large rats and bats, - that was over the top.
But as an action-adventure movie that ignores all of the historical background and just plays around with a famous character, it's good. It's original, fast-paced and entertaining.