Why do the boys find Castlerock very appealing, and what does Castle rock look like exactly?

The narrow wall of rocks forms a bridge between parts of the island, reaching the open sea. The area distracts the boys, who spend time rolling rocks around the bridge. Jack notes the rock could be a fort.

The tail-end part, where the rocks are all piled up. I've been near there. The rock makes a sort of bridge. There's only one way up."

There were only a few more yards of stony ground and then the two sides of the island came almost together so that one expected a peak of headland. But instead of this a narrow ledge of rock, a few yards wide and perhaps fifteen long, continued the island out into the sea. 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 they had seen from the mountain-top. The rock of the cliff was split and the top littered with great lumps that seemed to totter.

On the right hand was the lagoon, troubled by the open sea; and on the left-- Ralph shuddered. The lagoon had protected them from the Pacific: and for some reason only Jack had gone right down to the water on the other side. Now he saw the landsman's view of the swell and it seemed like the breathing of some stupendous creature. Slowly the waters sank among the rocks, revealing pink tables of granite, strange growths of coral, polyp, and weed. Down, down, the waters went, whispering like the wind among the heads of the forest. There was one flat rock there, spread like a table, and the waters sucking down on the four weedy sides made them seem like cliffs. Then the sleeping leviathan breathed out, the waters rose, the weed streamed, and the water boiled over the table rock with a roar.

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"We want smoke. And you go wasting your time. You roll rocks."

Roger shouted. "We've got plenty of time!"

Ralph shook his head. "We'll go to the mountain."

The clamor broke out. Some of the boys wanted to go back to the beach. Some wanted to roll more rocks. The sun was bright and danger had faded with the darkness