Montana allows the use of force to protect one's self or others from immediate harm, as long as the person believes it necessary to prevent serious injury or death. Force can be used in defense of one's home if the person believes the aggressor is trying to force their way inside. It does not seem to include the use of deadly force in self defense.
Castle Laws are laws that address the use of force when defending one's self inside their home, or on their property. Some states expand this to vehicles, and the person's place of work. Castle Laws generally include: the places where this law applies, the requirements fro use of deadly force, if there is a duty to retreat, the amount of force that maybe used in defending one's self or others. These vary widely from state to state.