Ohhh, ok, so not the size of a city, but I had a custom campaign setting with something like this. There was a floating oasis that the party was going to take across this dying world to help bring life to a the towns and cities out in the wastelands. The oasis had four massive stone pillars inscribed with complex magical rune spells to lift it up and keep it floating! But it also had massive pack-animals pulling it along, the magic only kept it in the air. I remember calling them saytoxen (or a saytox for singular), they were massive mammoth-sized beasts, a bit like elephants, but no tusks. They're a largely brownish color with a long green stripe down the back, a sort of magical mutation/adaptation to the dying environment. It allowed them to go long periods without food by gaining some energy from sunlight.