The door had a very nice looking Abjuration aura around it. Nothing dangerous that would cause harm to some one trying to open it, but it would defiantly put a stop to some silly wizard getting in with something like a simple knock spell. Apart from being protected from spells lower than 6th it also was protected from decay and the elements. It must have cost the bandits an arm and a leg, or maybe they just got it with a five finger discount.