This was supposed to be the first installment of a planned six-film series. Those plans were scrapped after it bombed at the box office.
Charlie Hunnam had lost 20 pounds for the last season of Sons of Anarchy (2008). During auditions, director Guy Ritchie was very bothered by his look, though he liked his performance, and asked him four times during the process about his poor physique. Hunnam said that when Ritchie brought it up the fourth time, he told him, "Look, dude, you keep bringing this up, the physicality. It's obviously your primary concern. So if you want to do away with all this auditioning bollocks, I'll fucking fight those other two dudes. I know who they are. You can bring them both in here. I'll fight them both. The one who walks out the door gets the job." Hunnam won the role.
Charlie Hunnam promised that he would get into incredible shape for this film, even saying he was going to do 500 push-ups every day, as he wanted to "look like one of those guys who do that." After filming concluded, Hunnam revealed that he did actually did several hundred push-ups every day, with his personal record being 1,050 in a single day.
Guinevere was originally going to be a part of this movie, as Arthur's love interest. However, Guy Ritchie said he couldn't give that story line enough screen time, so the character was rewritten and became Mage.
The film doesn't feature Lancelot or Merlin, intending to save them for a sequel. For similar reasons, all references to the Mage actually being Guinevere were removed during post-production.