The Imperialism policy that gives a free factory and a lump sum of gold when conquering a city is way OP and easily exploited. I got 40,000+ gold from taking 5 or 6 mid to large size cities... an insane amount. And now for the exploit: taking 4 or 5 cities with no walls or anything (maybe just conquered), give them to some minor civ you don't care about, then declare war on them in the same turn. Easily take all of the cities you gave them that turn. Gain 20,000+ gold this way. Rinse, repeat as often as desired.