Barrier Cities began as military troop facilities which were built seventeen years after the Leonid Meteor, which carried the Phantoms which fell on the Earth. A total of 130 facilities were built across the world as a base operation for the troops of the United States Military Forces (USMF) in their combat against the Phantom threat. However, due to the development of the Zeus Cannon in 2055, some of the Barrier Cities were not completed until two years later.