In my version of the book, the first description of castle rock comes when Ralph, Simon and Jack are exploring the island and they get to the top of the mountain. As they look out over the whole thing, Golding describes the castle rock end of the island as "another island; a rock, almost detached, standing like a fort, facing them across the green with one bold, pink bastion."
It is described again briefly on page 146 when the boys are discussing where the beast might be and then they decide to head down to that side of the island to hunt for it.
There is a further description on page 148 as the boys approach the rocky end of the island hunting for the "beast." There is further description on pages 149-153 as the boys move around castle rock and start to see all the different parts of it. The description ends there as the boys quickly go back to try and get the smoke going again on the island.