The use of deadly force for self defense is legal when "the actor believes that such force is necessary to protect himself against death, serious bodily injury, kidnapping or sexual intercourse compelled by force or threat." (§ 505(b)(2)). Any force used against the aggressor must be deemed necessary by the person in danger. There is no duty to retreat inside the home or work place when threatened.
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.