What was the castle on page 104?

Castle Rock is a small rocky outcrop which is connected to the 'pointed end' of the main island by a narrow causeway. When Jack leaves the group and forms his own 'tribe' he chooses Castle Rock as his headquarters because it has rocky overhangs which provide shelter, there is a spring which supplies running water and Jack believes that it is easily defensible against Ralph's group and the 'beast.'

Castle rock is the name that the boys give to a rocky outcrop, almost a small island, which is connected to the pointed end of the main island by a narrow causeway. After Samneric claimed to have been chased from the mountain top by the beast the boys decide to hunt for it. As Jack claims to have explored the rest of the island without ever seeing any sign of the beast the boys assume that it must be using castle rock as its lair, as this is the one place the boys have never explored. When they reach it Ralph goes alone across the causeway, feeling that it is his duty to do so as he is the chief. Jack soon joins him (probably so as not to lose face before his hunters). Once they are certain that there is no trace of a beast the other boys also come and explore. Ralph is dismissive of the rock, calling it a 'rotten place.' Jack is excited by its prospects, finding a trickle of fresh water to drink, an overhanging ledge of rock to provide shelter and a number of boulder on the top of the rock, which he declared could be tumbled with levers to defend the rock from attackers. Later in the novel, when Jack has formed his own tribe, he makes castle rock his headquarters.