The Super Barbarians

From the back cover of the Ace first edition: "The Acre was the only part of an entire world where Earthmen were allowed to live as they pleased and as they were accustomed. For elsewhere on Quallavarra, humanity was forced into servitude by the Vorra, The Super Barbarians who had somehow managed to conquer space. But within the Acre, the underling Terrestrials had cooked up a neat method of keeping their conquerors from stamping them out altogether. They had uncovered a diabolical Earth secret that the Vorra couldn't abide - and yet couldn't do without."