Some time in the past the world was controlled by two powerful cultures that ruled the world from flying cities. At some point a war broke out and a winner arose, but long before that happened they used the primitive surface dwellers below to fight their wars.
The world is still recovering from the damage done by the flying cities war machines, alchemical weapons, and bombs.
On the surface people can make a good living scavenging relics of the war. The world below is now in a steam era, with its own land ship’s, alchemist, and piloted war machines. Meanwhile in the cities above war has come again, and will be fought both above and below.