I wonder why there's a city in the ship anyway? I guess the aliens are migrating to Earth immediately, rather than wage a war for a while with the citizens back home. Maybe the aliens had already done some recon on Earth and realised it would be an easy victory over tiny insects.
But they didn't know about gantz?
Also, are these aliens in any way related to all the other aliens we've seen during the story? Have they met any of them? Is there an intergalactic neighbourhood or committee?
Is this whole damn thing happening inside a matrix anyway?