Lord of the Flies, by William Golding, is set on a tropical island on which a group of English schoolboys is living after they survived a plane crash. The first place they all live is near the beach. Ralph and Simon, with minimal help from anyone else, have built some shelters there, though only one of them is very sturdy. The shelters are near fresh water and fruit trees, as well as the ocean, of course, and the jungle foliage of the island is nearby.
The fort is on the mountain. In chapter six, most of the older boys are hunting, and Jack sees something nearby which captures his attention.
He led the way over the rocks, inspected a sort of half-cave that held nothing more terrible than a clutch of rotten eggs, and at last sat down, looking round him and tapping the rock with thebutt of his spear. Jack was excited. “What a place for a fort!”
Ralph does not share his enthusiasm. The spray from the ocean gets them wet, and he grumbles that there is no fresh water. Jack sees "a long green smudge half-way up the rock" and discovers a trickle of fresh water. The two of them keep climbing the pile of rocks until they reach the pinnacle.
A hundred feet below them was the narrow causeway, then the stony ground, then the grass dotted with heads, and behind that the forest.
Jack is thrilled to note that, from up here, giant rocks can be levered and dropped down onto any approaching enemy (which of course is what Roger does to Piggy several chapters later). Ralph's opinion is unchanging. He insists that "this is a rotten place.”
The words shelter and fort imply a sense of protection for those inside them; however, the reality on this island is that the enemy is within, so neither of these places provide safety for the boys.