It's ambiguous, and varies from state to state (the variations usually relate to side issues like whether you have a duty to retreat instead of shooting, etc.)

It's ambiguous, and varies from state to state (the variations usually relate to side issues like whether you have a duty to retreat instead of shooting, etc.) but the general rule is that the response has to be 'reasonable' and proportionate to the reasonably perceived harm. The Castle Doctrine doesn't, as far as I know, have a single clear meaning, but generally refers to rules that expand the circumstances in which a homeowner can use force (including deadly force) on an intruder to their home.