Originally posted by bbarrYou also can't be in check to castle (Like Uncle Adam said), or have moved the king or castleing Rook (Rook to be castled with). You also won't see the rook move til after the game continues... looks strange but is true.
Your King can't move through a space where you would be in check were your king to land there. This is in addition to the constraints listed by Uncle Adam.