Voodoo Planet by Andre Norton — Read

Lightning played along the black ridges above them, and below was a sheer drop to a river which was only a silver thread. Under their boots, man-made and yet dominating the wildness of jungle and mountain, was a platform of rock slabs, fused to support a palace of towering yellow-white walls and curved cups of domes, a palace which was also half fortress, half frontier post.

Dane set his hands on the parapet of the river drop, blinked as a lightning bolt crackled in a sky-splitting glare of violet fire. This was about as far from the steaming islands of Xecho as a man could imagine.