Castle Rock serves as the antithesis to the beach and the table rock where the boys first settle around and where Ralph builds the shelters. Since it is at the opposite end of the island and is surrounded by the angry sea, it symbolizes both the inaccessibility of the island and the low likelihood of rescue. Castle Rock also symbolizes the darker and more savage side of the boys.
Once Jack and his hunters take up residence at Castle Rock, it becomes the stronghold of the violent and savage side of the boys. Ralph and Piggy are afraid to go to Castle Rock even when they know they need to in order to try and get back Piggy's glasses. When they finally resolve to go and speak their minds to Jack, Roger tumbles the boulder from the cliff, kills Piggy and smashes the conch, signifying the end of the last vestige of civilization and order on the island.
So, while the beach and the shelters belong to Ralph, the idea of rescue, order and signal fires, Castle Rock belongs to Jack, the hunters and the release from all rules and civilization.