The so-called "castle rock" becomes a place for the hunters and serves as a stark contrast to the softer, smoother part of the island where the boys first congregate. Though the first discussion of it among the boys suggests that it could be a place for a fort, it really becomes a symbol for the fear and emptiness that fills the boys.
Some suggest that it serves this function because the ocean is so wild and unruly around it, it reminds them of the fact that they are truly at the mercy of forces far more powerful than themselves. And once it becomes the place for the hunters when they split from the other boys, it is where they truly begin to lose what traces of humanity they have left.
It is the place where they consider themselves safe from the beast because it is difficult to get to except by a single path. But, in the end, it serves to stand for the real beast which is the fear within them.