As Ralph walks across the bridge, or causeway, it seems that "Soon, in a matter of centuries, the sea would make an island of the castle." Comment.

I think that Ralph is realizing this island is not as nice and as safe as he once thought. The way Golding describes the rock castle shows that the island's charm is impermeant. He uses the words chasms and narrow neck, all of which can be used to describe something ominous.