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)
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)
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)
Answer: Most of them are Friesian or Friesian crosses but but some are Andalusian.