After you take some enemy cities, send troops out and cut the rail lines and roads. If they want to start the war back up they will fly into your territory. Double-edged sword really, cause it cuts down on your invasion time, but you can rebuild later. Also, if you have the cash, send spies into an enemy city and have them incite disorder, then buy the city for half price. Plus you get all the troops and workers associated with the city.
An addition to the wonders Meepa mentioned. Mid game try hard to get the one that upgrades all you troops, leonardos workshop I think it is. And adams smiths trading wonder will free up a lot of cash so you can inprove your research.
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