Finding and opening the new Demon Door is relatively easy. By chance if you've had trouble locating the Demon Door or figuring out how to unlock it and get inside, here are some tips to get the job done.
To find the new Demon Door in Traitor's Keep, stand in front of the keep's entrance and look to the right. You'll spot some stairs and should immediately go down them and through the hole in the wall at the bottom. Once through the hole, head straight ahead and down a small hill. Keep glancing over toward the left and you'll spot the Demon Door.
Getting the Demon Door open is as easy as looking the part of a criminal as opposed to playing the part. Wear the Prisoner Guard Outfit while interfacing with the Demon Door and it will open right up to reveal some neat spoilers inside.
As for where to find the Fable 3 Prisoner Guard Outfit, watch the YouTube video below for a quick video tutorial.