Zarathustra Refugee Planets #1: Avengers of Carrig by John Brunner

Once the city of Carrig stood supreme on this planet that had seen settled by space refugees in the distant, forgotten past. From every corner of this primitive lost world caravans came to trade — and to view the great King-Hunt, the gruesome test by which the people of Carrig choose their rulers. Then from space came new arrivals. And with them came their invincible death guns and their ruthless, all-powerful tyranny. Now there would be no King-Hunt in Carrig, or hope for the planet — unless a fool-hardy high-born named Saikmar, and a beautiful Earthling space-spy named Maddalena, could do the impossible . . .