Factual error: The Vikings raided England in the 9th and 10th centuries. The first castles were not built until post 1066. The castle in the film has rounded towers, crenellations and machicolations, which would date it at approximately 1300. That would tend to suggest that these Vikings were 400 or 500 years old, or that they had access to a time machine.
Revealing mistake: When Morgana's ship is being boarded, one of the Vikings features an arrow stuck in his body even as he boards. It is first (but not quite) kept out of sight by his shield, but when he enters the ship, he turns the shield away, and the arrow is suddenly visible. Somewhat hard to see by casual watching, so look carefully.
Revealing mistake: During the scene where the Vikings are attacking the castle, a soldier gets an arrow straight across the neck side on. Given that the Vikings were 40 feet below and head on to the ramparts, how on earth was that shot made?