Crypt-City of the Deathless One by Henry Kuttner

He remembered, dimly, what happened after that. It was like a series of fantastic visions, nightmare flashes of memory. At the bottom of the spiral was a cave, reminiscent of Chahnn and the other cities of the Ancients Garth had seen. Enigmatic machines loomed here and there. Unlike Chahnn, this city was lighted with the pale glow that came out of the walls and high-domed ceilings.

Cavern after cavern -- peopled with the silicate creatures, filled with the dead machines of the Ancients! And, finally, an immense cave, its floor slanting up to a raised dais at one end. On the platform a throne of black metal stood, and seated upon it was a gigantic four-armed robot, larger than any Garth had ever seen before -- standing, it would have been twelve feet tall, he judged.