The statue that Marcus glances at at the fort before the first Celt attack is a bust of Antoninus Pius, who was emperor of Rome at the time this film is set, AD 140. His reign is considered one of the calmest in Rome's history.
The main character's name is Marcus Aquila. Aquila is the Latin word for "Eagle".
Because of budgetary consideration, cast and crew had only one night to shoot the night battle scene, which gave it the disorganized, raggedy look they wanted.
When the troops are being awakened before the night attack on the fort, one of the soldiers complains about the helmets. As he laces the cheek plates, an injury is visible on his chin. This is derived directly from contemporary accounts of helmet laces cutting into soldiers' chins being a common complaint.
Historical advisor for the film, Lindsay Allason-Jones, gave a talk to Newcastle University Archaeology Society about her experiences working on the film. She explained that the production company decided to use stirrups, despite their being anachronistic, because there would have been issues getting insurance for the lead actors, especially as Jamie Bell had never ridden before.