In Lord of the Flies chapter 6 when Jack follows Ralph up the rocks what does he think the castle rocks would be good for?

Jack thinks that castle rock would make a good fort. He finds a trickle of running water under which he says, a coconut shell could be placed. Jack also though that the overhanging rock ledge would provide shelter and that the approach to castle rock, across a narrow causeway of rock would be easy to defend against atatckers or the beast, especially by using the boulders on top of the cliff as weapons.