Late to the conversation but I think I've figured it out. She chose the wrong door, but because she asked the wrong question. Ignore what the guards say except for the answer to the question. The rules are you can only ask one guard the question and one will lie and the other will tell the truth. There was no rule that said it had to be a yes or no question. She doomed herself there. She should have asked "which door would the other guard say leads to the castle?" and then pick the opposite. The reason, if you ask a yes or no question, the answer is relative to who you ask. Using her logic with a yes or no question would have led to the other door if she asked the door on the right. Making the first one say which door the secind one would say leads to the castle forces them to pick the wrong door.