Why mobile cities by the way?

The landscape is not dystopian, so I do not understand the need for these mobile cities. That said, the movie is pretty, the acting is decent, and the story elements were pretty good. Everything just seems grander for no good reason other than spending tons of money. Also, why not just make this movie rated R? Thousand of people are killed in this. One scene involves a man's head being blown off followed by a sword fight that leaves a bunch of dudes dead. This is already carnage, just rate it appropriately. The nuclear robot was cool though.

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