Other mistake: Arabic inscriptions can be seen on the golden desk, but the letters aren't connected like normal abjad. This is likely a computer rendering error since some editing software don't natively support foreign characters. (00:17:10)
Other mistake: When Vlad is in the armory of the monastery, he is standing directly under sunlight. This should have caused his skin to burn immediately and give away that he's a vampire. (00:52:05)
Answer: It was very common in the Middle Ages for those who ruled over an area to demand various forms of tribute, a very common one being soldiers or children to train as soldiers.
Greg Dwyer