"Koshtra," by the way, was a nickname of mine in college. Ultimately it derives from E.R. Eddison, The Worm Ouroborus. Lord Juss has a dream about his missing brother, in which an owl carved on his bedpost tells him to "Inquire in Koshtra Belorn."
Koshtra Belorn, as it turns out, is remote mountain, which the heroes have to find, and climb, in order to rescue Lord Juss's brother. Eddison, who wrote in the 1930's and 40's, was a climber, as well as a scholar of Greek and Old Norse, and a writer of gorgeously purple, pseudo-Elizabethan prose, and something of a right-wing crank in the Yeats/Eliot/Pound line. I love his books, especially A Fish Dinner in Memison, but I've learned not to recommend them to people. An acquired taste which most people are not going to acquire.