If only the front side of a door needs a key, the back side can be opened by monsters as normal. However, in certain cases involving thin doors, monsters may be able to open the door from the locked side (semi-locked). All monsters can do this, but only arch-viles, arachnotrons, cyberdemons, demons, spectres and revenants can open doors thicker than 7 units. This is because they are faster than the other monsters (the faster a monster is, the thicker semi-locked doors it can open).