The Demon Door claims to have once been a man that ate lots of meat, and now that he is made of stone, he can't eat. Coincidentally, a nearby headstone reads, "Luke Pressley. Overdosed on meat." This is the tombstone for the English game scripter Luke Pressley, who helped on the Fable II game.
To gain access to the realm, the Hero must kick a chicken at the door or give it a Crunchy Chick. There are several chickens near the door, and any one will satisfy the requirements to open the door.
Upon entering, the Hero will notice that the Forgotten Keep is, as the name states, an old forgotten keep, or castle. Oddly enough, it happens to reuse the Chamber of Fate's model.