It's sunset on Krypton. The towers of Krypton grew organically, like crystals, but then humans carved and polished them afterwards. That's the theory that concept artist Jason Dunn brought to a Krypton flashback in a recent episode of Smallville. This mix of the organic and the hand-crafted informed Dunn's sketches and matte painting for the episode.
This "cathedral-like structure, in the middle of a vast wasteland," would be similar to Clark's Fortress of Solitude, says Dunn. But it wouldn't be as chaotic looking, because the Kryptonians carved and buffed these structures after they grew out of the rocky landscape. You can see more pics, plus a movie of the final results, over at Dunn's site.
Planet Krypton Concept Art [Hawkeye's Sketchblog]