If they really wanted to make it right then Keira Knightley should have fought either topless or naked, because both were not uncommon among the celts and trhe saxons.
she actually did do part of the battle scene topless... unintentionally. Apparently, she had a 'wardrobe malfunction' during the filming, and her bikini top slipped upward off her breasts, leaving them exposed to the world for a while, and allowing several clandestine pics from this glorious moment to make their way onto the internet...