The features of the island are described as Rainsford is washed up on the island in thevery beginning of the story. There are jagged cliffs on the shore and a dense jungle surrounds the island; this is evident in the following description: "Dense jungle came down from the very edge of the cliffs. What perils that tangle of trees ... might hold for him...." The mansion is on a cliff near the coastline. Although, the author does not say which side of the island the mansion is on, he does say, "...on three sides of it cliffs dived down to where the sea licked greedy lips in the shadows." This is telling you that the terrain surrounding the mansion is rough and dangerous.
The shape and location of everything on the island is never explained. However, one could assume from the lighthouse that "tricks" shipman into sailing towards it that it is on the front part of the island where the jagged rocks are that trap and destroy their ships. The island itself is a fictional place somewhere in the Caribbean. The chateau is quite a hike from the jagged rocks where Rainsford arrived. It's probably somewhere in the middle of the island so that men who arrive will be worn out once they arrive. The Death Swamp is somewhere inbetween the two, perhaps off to the side of the island since Rainsford didn't encounter it until the "hunt" with Zaroff.