Ursula K. Le Guin 
Planet of Exile 

The Earth colony of Landin has been stranded on Werel for ten years — and each of Werel’s years is over 60 terrestrial years! After so long an exile, the lonely and dwindling human settlement is beginning to feel the strain. Every winter — a season that lasts a decade and a half — the Earthmen have neighbours: the humanoid hilfs, a nomadic people who only settle down for the cruel cold spell. The hilfs fear the Earthmen, whom they think of as witches, and call the farborns. But both peoples have common enemies: the hordes of ravaging barbarians called gaals, and eerie preying snow ghouls. Can the hilfs and the farborns overcome their mutual suspicions and join forces? Or will they both be annihilated?