Neil Marshall shot the film in such hard conditions that some cast members suffered health problems. As written in the Independent: Noel Clarke recovered from frostbite. JJ Feild quit smoking because of hypothermia. Michael Fassbender survived shirtless sprinting in sub-zero temperatures to become the next Daniel Craig.
The working title for the film was "Ninth Legion". The story goes that the Ninth Legion marched to Scotland from York with 3000 men and were never seen or heard of again. Historians dispute what actually happened to them. Some believe that they were disbanded, while others believe that they were massacred.
Neil Marshall and his director of photography Sam McCurdy spent about two years discussing the look of the film before making it. One thing they were adamant about was that it should be shot on location and nowhere near a green screen.
German archaeologists have found evidence of the 9th Legion on the banks of the Rhine River and carbon-dated them long after these events took place, suggesting that rather than being wiped out, the reason that there is no evidence of the 9th Legion being in Scotland after these battles is that they moved to Germany.
The Picts in the film are speaking Scots Gaelic. No written record of the Pictish language exists, but historians believe the modern language most similar to theirs is Welsh. However, director Neil Marshall thought viewers would be confused by a tribe in Scotland speaking Welsh.
In the original script, it was revealed that Etain (Olga Kurylenko) was the half-sister of Arianne (Imogen Poots) and it was she who had scarred Arianne's face. After the battle at the fort Etain escaped, and when Quintus returned to find Arianne at the end of the film there was a brutal fight between the three of them in which Arianne eventually killed Etain. This story thread and ending was removed by the producers prior to filming.
The Picts were generally thought to not have worn any clothes. However, seeing as the film was being shot in the wilds of Scotland in the depths of winter, it was decided that they should wear something.
Prosthetics designer Paul Hyett was thrilled to be part of the production as it afforded him the chance to make slashed throats, chopped arms, decapitations, head slicings, arrows in necks, axes in necks, burnings and head crushings. He ultimately went through over 200 liters of fake blood.
The Eagle (2011) can almost be seen as a sequel to this film, as it picks up the same storyline about the 9th Legion some 23 years later when the general's son comes back to find the truth about the Legion's disappearance and to collect the Eagle Standard.