The Super Barbarians — John Brunner

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 has somehow managed to conquer space.

But within the Acre, the underling Terrestrials had cooked up a neat method of keeping teir conquerors from stamping them out altogether. They had uncovered a diabolical Earth secret the Vorra couldn't abide - and yet couldn't do without.