Remember that the climax in terms of the structure of the plot of a novel is defined as the key scene in the story - that tense or exciting or terrifying moment when our emotional involvement is greatest. During the climax we learn what the outcome of the conflict is going to be. The climax is also the penultimate part of the plot structure coming as it does just before the resolution.
So, when we think about this great novel, for me the most tense, exciting and terrifying moment comes towards the end of the novel in Chapter 14 and 15. What we see is a massive collision of all the different parties in the novel with David, Rosalind and Petra caught in the middle. Having fled Waknuk and having sought safety in the Fringes, we find out that in a sense they have gone from the frying pan into the fire. David's uncle wants to kill David and keep Rosalind for himself to breed children. They clearly have not found the safety they wanted. However, at the same time, David's father is heading an armed force that is invading the Fringes to capture David and Rosalind and also kill as many "deviants" as possible. But, as if this was not exciting enough for us, we are also desperately waiting the arrival of Petra's friend for Sealand, who is coming to help David, Rosalind and Petra escape. Phew! Are you sitting on the edge of your seat yet? You should be! The resolution of course focusses on how these various conflicts are played out and ended, leading to the end of the novel.