After Tony frees Arthur (who by the way is tied with ropes and not chained as Tony was) his tunic is conveniently nearby to put on but not the long sleeved shirt that goes under it. However later Arthur is shown wearing it under his tunic.
King Arthur uses a rope to get to Guinevere. The rope is fixed to the opening when it should be hanging free of the castle.
Arthur is wearing an orange and red outfit in the watchtower, however, when climbing down from and then back to the tower on a rope his outfit changes to a white shirt (due to the fact that the climbing shots are actually stock footage taken from Prince Valiant (1954)).
The outlandish horns on the helmets and hats of the "Vikings" have no basis in fact. Helmets worn by Vikings were of a simple conical shape, occasionally featuring nose-guards and ear-guards. The purpose of a helmet is to protect the head. A projection such as a horn would catch, rather than deflect, a downward blow, snapping the head violently to one side and breaking the wearer's neck. Pre-Viking era priests and shamans sometimes wore headdresses festooned with antlers, which gave rise to the mistaken notion that their helmets were horned. The notion that Viking helmets had horns seems to originate in 19th-century German theatrical productions.
When the Viking leader sentences Tony, Doug and Arthur to death, they wouldn't have used an archer. Why would they waste the arrows when running them through with their swords would have been more practical?
Merlin "freezes" time in the control room in the beginning yet the lights on the instrument panels continue to blink on and off at their normal speed.
These events take place in 544 AD and has Vikings in England approximately 250 years before the Viking Age and their first recorded appearance in the British Isles. Historians date the beginning of the Viking Age as 8 June 793, the day of the first Viking raid on the abbey at Lindisfarne. In 544 the invaders would have been the Saxons from Germany, not the Vikings from Norway.
To find out from the Viking guard where Arthur is, Tony dunks his head twice under the water in the barrel then after the guard tells him he passes out. Two dunks under water shouldn't be enough to cause this. Additionally, the guard opens his mouth wide enough to reveal his modern dental fillings.
It is set in Cornwall Southern England but the still of a castle seen at the beginning, and towards the end, is Eilean Donan castle in North West Scotland.
Merlin turns the soldiers coming to the rescue into Vikings saying this was his last bit of magic, however he then magically disappears.
When Arthur returns from visiting Guinevere, the shadow of the boom mic can be quickly seen on the battlement just above Tony.