This ""mystery-adventure""--more the latter than the former and more nonstop than nonplus--follows two American boys en route to a school in Switzerland. On the plane they meet a M. Mistal, an Algerian; later they learn that he is dead, and that one of the boys has been used--in his pocket a bottle of perfume filled with diamonds. There are messages and threats to be deciphered: an avalanche (another lethal attempt) and the death of a professor, before the dark tower of the title provides the answer. This castle also has an oubliette which is for this kind of story which serves a need rather than a purpose. Of its kind, Just all right.