More about the history of Fort Black. The town keeps growing, under the guidance of The Fisherman. He's the total man in charge and he never dies or gets too old. Eventually it becomes a pretty fancy city, pretty big. And as Fort Black gets better, the sky city (which is called Aeo) gets crummier. Then, just as Fort Black is about to get larger than Aeo, Aeo falls down from the sky and a lot of people get killed. (We don't actually learn if something in particular caused the city to fall.) We also don't learn what was keeping it up in the first place!!!
Some notes: There's a reference here to a Maia Hilliers who gets killed up in Aeo. This is probably not the same Maia who is one of the archivist girls at the Fedema Global building, but again just guessing.