Expanded from Secret Agent of Terra (Ace Double in 1962.) This is one of 3 "Zarathustra Refugee" books: a loose series whose connection is that all take place on planets settled by refugees from Zarathustra - when that planet's sun went nova, 3000 starships carried a few million survivors into exile on a variety of worlds. 700 years later, the Corps Galactica, the Galaxy's police force, had pledged itself to a policy of non-interference with the ZR planets.

Once the city of Carrig stood supreme on this planet that had been settled by space refugees in the distant past. Caravans came from every corner of this world to Carrig- to view the great King-Hunt, the gruesome test by which the people of Carrig chose their rulers. Then new arrivals from space came to this primitive, lost world. None could withstand their invincible death guns or ruthless all-powerful tyranny. Now there was no King-Hunt in Carrig, or hope for the planet- unless a foolhardy high-born named Saikmar, and a beautiful Earthling space-spy named Maddalena, could do the impossible... unite the birds and colonists against the invaders.

(The other "Zarathustra Refugee" books are Castaways' World (a.k.a. Polymath) and The Repairmen of Cyclops.)