Andre Norton 
Defiant Agents [EPUB ebook] 

Operation Cochise: a carefully planned Western move to colonize a planet ahead of the Russians. Travis Fox had been an eager volunteer, but the morning he dragged himself half-conscious from the wrecked spaceship on the planet Topaz, he sensed the terror which would threaten the project. Travis never learned why the ship had crashed, nor why he and the other Apache agents had been shot into space without warning. But he quickly finds that the Russians already had a horde of barbaric Mongols on the planet. Now if Travis can’t unravel the secret of the eerie underground chamber in the towers hidden in a valley of mist, both Topaz and Terra could be destroyed.