Then I think if it were me, I'd do them fully at different times (unless you were trying to iterate technically), so you can get into the mindset of the culture. One city may be scratch built and be very orderly, while another may be a shantytown bicmac, while another may be built around a fort, etc
You could make a killing on Etsy selling Thunderbirds eyebrow sets.
The Mole is pretty good, too.