Castle Rock is defended by rocks at the bridge connecting it to the rest of the island.
Castle Rock is essentially an island. It has a lagoon on one side, and the ocean on the other. It is attached to the island by “a narrow ledge of rock” that is “a few yards wide and perhaps fifteen long” (Ch. 6). To the boys it looks like a castle, so they name it Castle Rock.
There lay another of those pieces of pink squareness that underlay the structure of the island. This side of the castle, perhaps a hundred feet high, was the pink bastion … (Ch. 6)
The rock is also dangerous and somewhat scary, but also interesting. This is one of the reasons Jack chooses it in the first place when he breaks off from the others and forms his own group. The fact that it is surrounded by cliffs and rocky shores with nothing but ocean beneath makes it very defensible.
When Jack has taken the island and Roger goes to the bridge, he is challenged. At this point, he sees how Jack and the boys have defended the island.
A log had been jammed under the topmost rock and another lever under that. Robert leaned lightly on the lever and the rock groaned. A full effort would send the rock thundering down to the neck of land. (Ch. 10)
This way, the only people who are allowed into the club are the ones that Jack wants. He has guards posted at the entrance of the “bridge” and no one is getting in unless he lets them in.
Although it may seem like boys playing war, what is happening here is serious. Jack has formed a savage and cruel society, but in many ways it is more ordered and structured than Ralph's was, despite the fact that it represents the unraveling of civilization. He craves power and his stronghold on Castle Rock demonstrates that.