Conan - The Devil in Iron by Robert E. Howard

Along the dark, silent tunnel Conan groped, momentarily dreading a fall into some unseen pit; but at last his feet struck steps again, and he went up them until he came to a door on which his fumbling fingers found a metal catch. He came out into a dim and lofty room of enormous proportions. Fantastic columns marched about the mottled walls, upholding a ceiling, which, at once translucent and dusky, seemed like a cloudy midnight sky, giving an illusion of impossible height. If any light filtered in from the outside, it was curiously altered.

In a brooding twilight, Conan moved across the bare green floor. The great room was circular, pierced on one side by the great, bronze valves of a giant door. Opposite this, on a dais against the wall, up to which led broad curving steps, there stood a throne of copper, and when Conan saw what was coiled on this throne, he retreated hastily, lifting his scimitar.