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.