After the uncontrolled industrialization of Duro and the devastating attacks of the foreign invaders, the land of Duro became uninhabitable, and its ecosystem became pretty toxic. This led Duros to construct or modify the shipping docks to develop enormous ships that could house hundreds of thousands of people. These ships, aka orbiting cities, were covered with domes to ensure no toxic substance would reach the population. These city-ships were twenty in number, and most of the Duros lived in these cities. Still, the uninfected area of Duro was reserved for farming, which also housed a small number of Duros.