Man Who Loved Mars, by Lin Carter (epub/Kindle/pdf)
Once it had been king city of a mighty empire and the center of the ancient faith; Gateway to the Gods, the old epics name it. Now it was dead, empty, deserted, only a dim ghost of its vanished splendor. Such was Ilionis, the Lost City of Mars. A somber ruin, cold and lonely. But Ilionis was not forgotten. The old city held a valuable treasure...a treasure that brought Earthmen Ivo Tengren and scientist Krensny on a strange and difficult journey to the city's gates. And now an even stranger journey was about to begin....