Other mistake: At Kerak, as the 2 armies are facing off, the wind is blowing the banners of Saladin's army hard to the left, but as the view switches to Baldwin's army, facing the opposite direction, their banners are also blowing left. (01:33:00 - 01:33:50)
Other mistake: When Balian talks to God on the hill where Jesus died, after he says his line, the next shot is him sitting down overlooking the empty valley below. A white light travels from right to left going 45-50 mph. Nothing back then could go that fast, and even if anything could, a torch is yellow and red, and even if it could be white, no rider can ride that fast with a white-light torch and keep it from going out - leaving us to assume it's a car.
Chosen answer: As Sereenie said in the corrections, Balian has spent months travelling with his father, during which time he has learned many things, including sword-fighting, and how to read and write.
STP ★