An Iron Door is only able to be toggled by a switching mechanism such as Levers, Pressure Plates or a Button. If an iron door has at least one "open" signal connected then it will be "open". However, a clever engineer can make it so that the door blocks passage when "open" and allows it when "closed". This creates the possibility to make doors that are only controllable from remote locations using a switch and Redstone; otherwise putting a redstone torch in front of the door will open it.