There's two areas isn't there? Nightmare Frontier and Nightmare of Mensis (The latter of which has two bell maidens).There's only one mandatory area that contains Bell Maidens by default, which is absolutely ridiculous. Also, I think the "Play Offline" option is far too enticing for players who dislike the invasion mechanics. People who have had a rough time with invaders in previous Souls games would see that and probably not even think twice about going online.
Edit: Oops, missed the mandatory part.