The actors did their best with the material, and the actors were a fairly sterling bunch: Robert Taylor, Mel Ferrer, Ava Gardner, Stanley Baker, Felix Aymler to name just the A-list lot. The smaller roles were all played by stock actors familiar from many films of the period.
But it was the battle scenes that made it a laughable piece from today’s perspective. The big battle between Arthur and Mordred was so firmly based on Laurence Olivier’s wonderful battle sequence in Henry V that they might as well have used the same footage. Until it came to the two sides meeting. At that point, things fell apart badly. Nobody was really doing any fighting. Swords flashed, but they seldom hit anyone. There was quite a bit of noise, but it was all noise. And the extras. They had to be seen to be believed. Blokes dressed up in silly outfits pretending to fight, and unfortunately being caught on camera pretending – and ambling along without any real effort.
One guy’s ‘fighting’ consisted of jumping up and down in the saddle. There was no passion, no guts to the thing. When Arthur’s backup group of archers got into the story, they shot their arrows off towards the passing knights on horses, and most of the arrows fell visibly within a few yards onto the ground. One or two knights fell down dutifully off their horses, but certainly nobody was actually hit with an arrow.
And later on, Lancelot and his troop walked into a trap set by the Picts. Lancelot basically carried on talking to those around him while the extras in the background did their pretend fighting. Because he was a main character he knew he had no problem with getting hit with anything, so he didn’t really need to take care of himself. And I’m sure the guy who did all the bobbing up and down in the saddle was there in the background again, jumping up and down while fighting nobody. Incredibly one knight even thrust his sword into a passing Pict and the sword very obviously went in under the Pict’s arm and out behind him: no one has ever died of having a sword thrust under his armpit as far as I’m aware, but this Pict did.
There's a review on a site called Monster Hunter which says pretty everything else I haven't said here.