The castle itself was constructed many years before the Moribund War, and acted as the capital of Anor Londo, the largest kingdom to exist at the time. Since then, the kingdom around it has fallen into the depths of the earth and the castle has been corrupted by Darkness' influence and has twisted into a form more befitting of its new owner.
Around the grounds of the castle roams a large beast known as Cerberus, said to be the God of Darkness' personnel pet, in reality however it is simply another demon the god has under his employ. Cerberus acts as a guard dog of sorts who keeps unwanted visitors from entering its masters home.
The castle is maintained by Margaret Ravencroft, who not only keeps it in perfect condition for her lord and his court, but also acts as its last line of defense if it were to be raided by the denizens of hell.