"Castle rock" is quite literally the castle-like rock formation that Ralph and the boys decide to explore at the end of Chapter Six of Lord of the Flies. The area is described as consisting of rocky ground with two sides of an island connected by a narrow ledge of rock. The side of the castle is described as being a hundred feet tall with a "pink bastion" and a top "littered with great lumps that seemed to totter."
With Ralph in the lead and Jack following closely, the boys explore the area. Jack proclaims, "What a place for a fort," and clearly sees the area as an advantageous space for any future battles. Ralph protests that the area is not well-equipped with necessities; there is no food, shelter, and minimal fresh water. With tensions rising, the chapter ends with the boys in a "mutinous" silence and with Jack leading the way across the bridge.