In Civ3, it's possible to specialize your cities, but most people don't know how to do it.
In Governor screen, there's a tab called Production. It's possible to set several priorities for production and choose this city, all cities and continental cities. If you need a settler factory, tell the governor to build only settlers (disabling all other unit types) and to produce growth (food). The same config could happen to build a worker factory, only enabling to build workers instead of settlers. A spearman factory could be done setting to build defensive land units and emphatizing production (shields). When you'll build a wonder, tell the governor to emphatize production, so it will try to get the most shields possible. Obviously, a good micromanagement do a better job, but it takes time.
Jesse Smith said "There are 19 town permits that have been added to the game." Maybe governor system in Civ3 was entirely redone. By the way, that Production tab has 15 options. I think most people here don't use/like these governors, but maybe they'll do a better job in Civ4.