Factual error: Hadrian's Wall is facing the wrong direction. The Wall was built to defend Britannia from the Picts in Scotland; therefore, its front should face north. However, as the Romans approach it from the south, they are greeted by the sheer face of the Wall.
Factual error: The movie supposedly takes place in and around 460 A.D. In the movie the 9th Legion in Britain is pretty much intact, only having recently been abandoned by Rome. This legion is young and strong enough to defeat a large army in the movie. However Rome abandoned Britain around 410 A.D. By 460 A.D. any original members of the 9th legion would have been in their seventies.
Revealing mistake: As Aurelius and Wulfila fight, one shot after Aurelius lifts a long-handled battle axe, we can see two shots with a jet contrail in the sky, which were impossible in the 5th century AD.
Factual error: Near the beginning of the film, the movie claims to take place in 460 A.D. but Romulus Augustus was not made emperor until 475 A.D. and deposed in 476 A.D. The events of the film obviously happen in a much, much shorter time frame and did not take 16 years.
Character mistake: Merlin tells Arthur that Pendragon means Son Of the Dragon when it actually means Top Dragon or senior leader. Pen is Welsh for top not son.