Dwarven Citadel, a floating chunk of rock that's slowly mined out by dwarves, sits besides its attendant "farm" asteroid, where trees and and food are grown for the mining colony. Once the "citadel" is mined out, it's towed away, and another brought here. Over centuries, the build up of water from the magical river on the farm and smoke and debris from the citadels, has built up a thick, breathable atmosphere in the region. Traders come to buy the dwarves fine metal goods, and bring more food, drink, fine luxuries for the bearded folk based on "spelljammer" a fantasy space setting for D&D :) this is an image I made a year ago but forgot to upload! doh