City Economy
The products which your cities extract from the surrounding terrain are the fountain from which your civilization is watered. There are three types of products: food points, production points, and trade points. The following sections describe each of these resources along with its properties, uses, and limitations.
- Food
Your population needs food to survive. Each citizen requires two food points per turn; in addition, some units (such as Settlers) may require food points from the city supporting them.
Every city has a granary for storing food points (the building called a Granary enhances this capability). Cities producing more food than they require accumulate the surplus in their granary, while those producing less than they require deplete their granary. When food is needed but none remains, the city population starves, killing food-consuming units first, followed by citizens, until the food deficit ends.
Excess food can increase the population: the city granary has a limited capacity, and once full the city grows by one citizen and the granary starts again at empty. But since granary capacity increases with population, each citizen is more costly than the last, making this mode of growth important only for small cities. (An alternative way for cities to grow is "rapture", described in the section on Happiness.
- Production
Production points (also known as "shields") represent manufacturing output. Most units require production points as upkeep, and demand them from their home city, although under autocratic regimes each city supports a few units for free. If city production drops too low, the units that cannot be supported are automatically disbanded. You can also disband most types of unit at any time. If a unit is disbanded while in a city, half of its production cost will usually be put towards that city's surplus.
Production points in excess of any required by the city's units are put towards whichever unit, building, or wonder has been selected as the city's current product. Just as food points accumulate in the city granary and yield a citizen when it reaches full, so production points accumulate until the cost of the product has been achieved. Products appear in their city when complete -- units appear on the map while improvements and wonders are added to their city's list of structures. Any leftover production points remain available to be applied towards the next project.
Some units, such as Settlers, are built from city population as well as production points; the city size will reduce when the unit is built. A city will not finish building a unit that would take all of the city's remaining population; in this case, production points will continue to accumulate until the city is of sufficient size.
A city can build at most one product per turn, regardless of its production surplus.
When a city has finished the work you have given it to do, it will try to build the last item again if possible, otherwise it will choose a new target itself. If a city is currently producing gold (building Coinage), the activity which never completes.
Each player is free to build any products that his technology has made available, with a few restrictions; see the sections on Units, City Improvements, and Wonders of the World for more information. Be careful -- the game even gives you the freedom to produce units you cannot support and buildings whose upkeep you cannot afford, both of which will be disbanded immediately after completion.
You can always change the product on which a city is working, though you lose half of the accumulated production points when switching from a building, unit, or wonder to a product from one of the other two categories. You can spend gold to complete a project in one turn by hitting the Buy button on the city dialog.
Cities with a large production output contribute to pollution, which affects the tiles around the city. See the Terrain help.
- Trade
Trade points reflect wealth generated in each city by external commerce. Some trade points may be lost to corruption, which varies among forms of government, and tends to increase with distance from your center of government. Each city distributes its remaining trade points among three uses: gold, in the form of taxes, goes into your national treasury; luxury points influence citizen morale; and science points ("bulbs") contribute towards the discovery of new technology.
You must choose a single ratio for your civilization by which trade points are distributed among these three uses. Though you may alter the tax rates on any turn, you are constrained to multiples of ten percent, and most forms of government limit their maximum value.
Having this single ratio does not impact gold and science, because gold and technological progress are both empire-wide tallies. Luxury is more problematic, however, because its effect is local -- it affects only the city producing it. Thus, while it would be convenient for unhappy cities to invest all their trade in luxury while others invested in science or taxes instead, you will instead have to compromise among the needs of all your cities (although there may be ways to make local adjustments, such as assigning citizens as entertainers). See the section on Happiness for more details on the effect of luxuries.
Besides working terrain gifted with rare commodities, or logistical benefits such as waterways or roads, you can increase trade by using units to establish permanent trade routes between two cities.
The game may limit the ability to trade or the yield from doing so depending on whether the two cities are on different continents and/or in different nations; for international trade routes, on the diplomatic status of the two trade partners; and for domestic routes, there must be a minimum distance between the two cities (nine tiles).
A trade route is established when a suitable unit (a Caravan or Freight unit) enters an eligible city. For your own or allied cities, you need to issue the "Establish Trade Route" command. This creates a trade route between the unit's home city (which might be different from the city that originally produced it) and the destination.
The origin civilization of the unit gains immediate revenue in gold and science from selling its trade goods at the destination city. The initial revenue depends on the trade already produced by the two cities involved and their distance apart. The destination civilization learns about the location of the origin city, if it is not already known.
The ongoing trade route that is established benefits both its origin and destination cities equally by generating trade points for each city every turn. The amount of ongoing trade increases with the size of each city and the distance between them, and may also depend on the factors mentioned above. The trade relationship also gives each civilization limited ongoing intelligence about the partner city. If a trade route ever becomes unsustainable -- it may be permanently canceled.
Each city can only support a limited number of trade routes. If you attempt to establish more, the trade route with the smallest ongoing revenue is canceled if it would be less than the new route. (Otherwise, the origin civilization can still gain initial revenue by entering the marketplace and selling trade goods, but it is reduced to a third.)
This limit is enforced only when attempting to establish a new route. If you temporarily lose the ability to establish as many trade routes as a city already has, existing ones are not affected.