When the boys first venture to Castle Rock, Jack is ecstatic about the potential of the place to be a fort; he looks at the clusters and boulders, viewing it all from the perspective of strategy and defense:
"'Shove a palm trunk under that [boulder] and if an enemy came--look!'
A hundred feet below them was the narrow causeway then the stony ground [...]
'One heave,' cried Jack, exulting, 'and--whee--!'" (106).
Ralph, however, perceives the cluster of rocks to be a "rotten place" (106). He shares none of Jack's enthusiasm for the place, choosing to stay focused on the possibility of rescue and a return to civilization.
As the novel progresses, Castle Rock comes to symbolize control and power. After Jack forms his own tribe, he relocates his hunters to Castle Rock, and the stony fort becomes the seat of Jack's power on the island. The boys' name for the fort, 'Castle Rock,' becomes incredibly fitting, as Jack's rise to power and total control over his tribe resembles that of a king; he has other boys tortured on his command, and the approach to the 'Castle' is unassailable.
In many ways, Castle Rock also represents the boys' turning away from civilization and their final descent into savagery; they have chosen to make the fortified cluster of boulders their home instead of focusing on the hope of rescue. With this in mind, it is fitting that Piggy, who for so much of the novel symbolized reason and order, meets his death at Castle Rock, the epicenter of Jack and the hunters' decline into savagery.