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Depending on where and how big your city is, it is profitable to build more buildings. The Aquaeduct and sewer system improve how big your city can grow, market/bank/stock exchange increase trade(I think), factories increase production etc... You can always select the building from the right list and click the help button to show you what it does.
Of course I always build things like city walls, coastal defense etc., but if I don't have to fear one of my cities being attacked(because it's already defended very well), I usually build barracks, harbor and airport first, then workers/engineers(to remove potential pollution), aquaeduct and sewer system (because I like all of my cities to be big), then production stuff, then food stuff, then trade stuff and some happiness always inbetween. In the end, when I have everything, I either go at unit produciton(if I'm at war), at money(if I want to prepare for the future), at caravans/freight(if the city's on the same continent as another one) or at a spaceship(yeah, f*ck you, allies!).
I usually have exactly five cities(because then traderoutes are used perfectly). These cities are, like, huge, because I make the land around fit perfectly. Then, when I go at war, depending on the settings, I either destroy the enemy cities, or, if I capture them, I use them as a troup supply outpost first(via airport), and then disband them(via building settlers). Every city less is a city less to potentially get captured by enemies. Though, If I know that I'll lose some city, I sell the buildings that will help the enemy the most(like defense stuff).
The way I think might be a bit strange, I mean, only 5 cities, but if I want to build ideally, no city areas may intersect, and I usually play island games, so there's not that much space.
Of course I could try to build multiple complexes, but... I have better things to do than to try that out right now.
My playing style is crazy, but I hope you can still use my advice
~ A.V. L.