You can ask only one question to go out from there. What should you ask?
Updated video with explanation
Explanation: If she asked the truth-telling guard which door the lie-telling guard would say led to the castle, he would truthfully say that the lie-telling guard would indicate the death door. If she asked the lie-telling guard which door the truth-telling guard would say led to the castle, he would untruthfully say that the truth-telling guard would indicate the death door.
So, when asked which door the other guard would indicate is the castle door, both guards would indicate that the other guard would point to the same door, the death door. If they were asked which door the other guard would say pointed to the death door, both guards would say that the other guard would point to the castle door. Thus, she knows which door to enter and not to enter.
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