Empty Quarter - Back when France was still a Kingdom (middle ages), the Serment family were asked by the King to prevent demons and evil spirits from coming and going. So the Serment placed barriers made of spiritual power.
While demons live in a different realm, there are places in their world that can easily connect to this world, such as the Empty Quarter in Colline Babel. The area must stay pure and so quite a the Exorcist family is living there.