The Black Citadel

You turn the cold brass door knob but the door doesn't open. You examine the ring of keys that you just took from the dead guard, but there must be about a hundred of them. Each one has some sort of markings on them like writing. You have no idea what key fits the lock here, and you don't want to spend half an hour trying them all so you have no choice but to go back the way you came to the t-junction.