Factual error: At the very beginning of the movie, the voice-over tells us it's around 300 AD. We see Roman soldiers, carrying shields and standards with the Chi Rho symbols. Those weren't introduced around 300 AD, but much later. Legend has it that emperor Constantine ordered his men to paint the symbols on their shields before a battle in 312, but Constantine's Arch in Rome, which depicts his victorious soldiers, doesn't show any of these symbols and the symbols in the movie are made of iron, not paint. (00:00:50)
Other mistake: The movie starts with scenes from a battle between Romans and Sarmatians. For a few seconds, we see a (very dark) shot of a warrior holding a severed head in his hand. That very same shot is used later in the battle between the Woads and the Sarmatian knights protecting the Bishop. Director's cut only. (00:01:05 - 00:08:30)
Continuity mistake: When the knights & the Bishop are around the round table, the Bishop's assistant is heard saying to Jols (after introducing Bishop Germanius), "A round table? What sort of evil is this?" and he is not on screen. Jols, however, is and he stands as though there is no one talking to him. In the very next shot he is looking at the assistant and speaks to him as though they were talking the whole time. (00:21:50)
Factual error: The Romans left England in 410 AD and the Anglo-Saxons didn't arrive in England until the late sixth century. Between these times the Britons were in control of the island. (00:23:55)
Plot hole: When the two Legionnaires open the gate of the Hadrian's Wall, they lean forward with their shoulders against the doors as if to push, but they are really pulling. (00:33:10)
Deliberate mistake: From outside, the dungeon where Arthur finds Guinevere has no windows. When the knights force themselves in, it is pitch dark. But in some scenes a window is to be seen in the background, although the jails are one floor down. Even better the cages, in which among others Lucan sits: they are lighted from below. (00:46:20)
Deliberate mistake: When Arthur follows Guinevere to the meeting with Merlin, her and the trees' shadows point at the viewer. Only Arthur has two shadows, one points to the left. The source of the moonlight is below the next ridge, and it's bright like a floodlight. Later, when Arthur is talking to Merlin, his face is always illuminated, even after a 180°-turn. (00:59:00)
Revealing mistake: When Lucan draws the ring of dead Dagonet's finger, you can see the finger move and keep its position. (01:14:00)
Deliberate mistake: When the horses shy to get their masters back to Arthur's aid you can see Bors pull the reins to provoke that action. (01:28:35)
Revealing mistake: Just before the battle of Maldon, the British spy climbs into the big tree and you can see the tree is fake. The spy isn't actually climbing the tree (by grabbing branches), he's using some kind of ladder. (01:36:55)
Continuity mistake: When Cedric holds wounded Tristan to show his triumph to Arthur, the underdog is standing (you see his head at the height of Cedric's), next (when he looks at his falcon) he is kneeling, and when Tristan receives his deathblow, he's standing again. (01:44:35)
Visible crew/equipment: In the last fight, when the first Saxons enter the door, there is a doorstop visible to keep the door open. (01:47:50)
Factual error: I normally wouldn't bother with this sort of nitpicking, but this film specifically claims to be historically researched - and it's full of historical blunders. For a start, the film is set as the Empire withdraws its last troops from Britain - which was in 407 AD. Now Artorius Castus was a real Roman officer who really did command Sarmatian foederati at Hadrian's Wall, but he died around 200 AD. Cerdic was a real Saxon warlord who did go raiding the Britons with his son Cynric, but he did this in the early 500s. Pelagius really was tried for heresy, but he was acquitted and died of old age; the trial was a decade after this setting, and in the fifth century you couldn't be executed for heresy anyway. Also in the fifth century the Pope had no authority over Imperial troops. I could go on and on but that will do for now.
Revealing mistake: When Arthur is giving his speech before the last battle, in the background are three radio towers. You never know Arthur might need a radio to find out tomorrow's weather.
Factual error: In the beginning, which is around 400 A.D., it shows that the saddles have stirrups on them. Stirrups were not introduced in Europe until centuries after the fall of the Western Roman Empire.
Factual error: Tristan's bird of prey is a Harris' Hawk - a species native to the South-West United States and Mexico, and thus unknown and unavailable in Europe at the time the movie is supposed to take place.
Continuity mistake: In the scene where the knights are sitting at the round table near the beginning of the movie one of the knights has a plus sign shaped scar on his forehead. In the next shot,the same knight now has a sideways cross shaped scar. The scar changes almost every shot throughout the scene.
Continuity mistake: The knights reach the outpost beyond the wall. One of the knights reports to Arthur that the Saxons are approaching. During the exchange he's shown with his hair pushed off his face, then its covering one side of his face, then its off again.
Factual error: All the Roman troops wear armour and uniforms appropriate to the 1st century AD (the "classic" era most often depicted in illustrations). The film is actually set several hundred years later, by which time Roman soldiers looked very different.
Continuity mistake: During the battle at the end of the movie, the first wave of Saxon infantry is sent through the open door in Hadrian's Wall, and then it closes behind them. Mayhem ensues. All but one of the soldiers escapes after the Wodes fire arrows at them while the Knights ride them down. The single survivor staggers through the opening doors (which open just a crack, enough for the guy to slip through), and then stands there, and in that shot we see the doors behind him are now closed. An instant later, it cuts to the hillside where the Knights are standing on their horses looking down at the wall, and we see the doors both standing wide open. Scene cuts back, and we see they're closed again.
Answer: I don't think it's supposed to be German. Probably Old Saxon. Could be something like "slahten fiand" - slaughter enemy.