There is no duty to retreat when you are in your home or office. Justifiable use of deadly force is permitted if the person feels that they are about to be killed, or that the attacker will greatly injure them.
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.