The climax of a story is where the conflict or problem is resolved. When Rainsford swims back to the chateau, the problem has not been resolved. Zaroff is still hunting him, and he still has to survive. The climax comes when Zaroff is surprised by Rainsford in his bedroom. Even though Zaroff tells him that he (Rainsford) has won the game, Rainsford says "I am still a beast at bay" (pg 11) That is an idiom that means he is still being hunted. He challenges Zaroff, and Zaroff says "One of us is to furnish a repast(meal) for the hounds. The other will sleep in an excellent bed." (pg 11) Zaroff then says, "On Guard" which is what is said before a fencing or sword fight. The Resolution or Denoument of the story is very short. It just says that Rainsford slept in the bed. That means he won the fight, and Zaroff furnished a repast for the hounds.
If you think that the climax is in the bedroom and the dénouement is about Rainsford sleeping in the bed, then where is or what is the falling action? (This question is based on the dramatic pyramid plot structure.)
I think so because Chateau is a dangerous place, and he swims back to it? I think that's the climax on wheather he'll escape alive or not.