Or in the distant future, with the artifacts being modern technology. Examples include Empire of the East, most of Vance's stuff, The Book of the New Sun, and any number of others. Sometimes you see riffs where it's on a spaceship (the Jacob's Ladder Trilogy) or a colony world (the Darkover series, Lord of Light). John Ring has a riff that catalogs the collapse of the high-tech civilization into magic-ish barbarism (the Council Wars). Mark Lawrence seems to be particularly fond of the trope, with all three of his published series to date (the Broken Empire, the Red Queens War, and the Book of the Ancestor) taking place in worlds where a technological civilization collapsed leaving behind wonders both magical and technological. For more examples than you could possibly need, see Ancient Advanced Humans, After the End, or Advanced Ancient Acropolis on TV Tropes.