Other mistake: The two headed monster's heads are pretty big. Big enough to swallow a person whole. But when Val Kilmer jumps on one to impale it, the sword comes out of the bottom of the monster's head. For this to happen, the sword would have to be unbelievably large, larger than a man, which he would be unable to wield anyway.
Other mistake: Near the end when they reach the castle, the drawbridge isn't even closed properly, they could easily climb up the side of the drawbridge and into the castle. What's the point of having a drawbridge that doesn't close.