The resources which your cities extract from surrounding tiles are the source which powers your nation's economy. Here each of the three resources is described.
Your population needs food to survive. Each citizen requires two food points per turn; each settler requires 1 food in Anarchy, Despotism, Monarchy, and Communism, 2 in Republic and Democracy, and 3 in Fundamentalism. per turn from their home city. Since all terrain squares without extra resource except grassland yield less than 2 food points, they cannot feed the citizen working them unless they are improved.
Every city has a granary for storing food points (not to be confused with the building called a Granary which enhances this capability). Cities producing more food than they require accumulate the excess 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 settlers first, followed by citizens, until the food deficit ends.
Excess food can increase the population: the city granary has a limited capacity, and when it reaches 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.
There are three buildings which enhance food production and population growth:
Every city generates at least one production point per turn. They are demanded first by any military units, settlers, workers or engineers supported by the city; each unit costs one production point per turn, though under autocratic regimes each city supports a few units for free. If city production drops too low, the units that cannot be supported are disbanded.
Points in excess of any required by the city's units are applied 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 buildings and wonders are added to their city's list of structures. Any leftover production points remain available to be applied towards the next project.
Each player is free to build any buildings, units, or wonders that his technology has made available, with a few restrictions: each city can have only one of each building; some buildings require that others be built first; and a few wonders can only be completed by one civilization per game. 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. Note that building settlers requires not only production points, but a citizen as well. A city cannot build a settler with its last citizen unless you enable this by adjusting its City Options, in which case the city will disband and turn into the settler.
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 (although if you change production the turn immediately after completing an item, you won't lose any of the production points that were left over from that turn). You can spend gold to complete a project in one turn by hitting the buy button on the city dialogue; the game allows you to confirm the cost before charging your treasury. The formula for units is: twice the difference between total production needed and the accumulated production points plus the square of that difference divided by twenty. The formula for everything else (buildings, wonders, etc.) is: twice the difference between the total production needed and the accumulated production points. (Depending on server version, the cost is multiplied by two or four if the project is a Wonder.)
The cost is multiplied by two again if production of this item hasn't started yet.
Should you need gold instead of production points, you can direct cities to mint coinage. Instead of producing a building or unit, the city will use its labor to produce one gold piece per turn for each production point it generates.
Several buildings enhance production. Note that each city can have only one of the four power plants (that is, only one of Hydro Plant, Power Plant, Nuclear Plant, or Solar Plant):
When a city completes a unit it normally starts producing another of the same type, and after completing a building or wonder, chooses a different one to build; new cities start work upon the best available defensive unit. Often you will instruct the city to work on something else instead, but this wastes time and attention if you already know the next several items the city should produce. In this case you can access the work list for that city and specify several products at once; the city will produce them in the order specified without any further intervention.
If you develop series of products you often use but tire of entering them manually into every city's work list, the game allows you to define named work lists and add them to city work lists. You might for example define a "coastal" list of improvements for cities on the ocean, or a "science" list for cities whose research output you are maximizing.
Trade reflects wealth generated in each city by external commerce. Some trade points may be lost to corruption, which varies among forms of government and increases with distance from your capital city. Each city distributes its remaining trade points among three uses: gold, in the form of taxes, goes into your national treasury; luxury points influence worker morale; and science points 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 this ratio 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.
Three buildings and one wonder directly affect trade directly, or indirectly by affecting corruption:
Trade routes are not available in the multiplayer ruleset. Many believe that micromanagement, luck in national positioning, and variability in diplomatic relations creates too much effort and imbalance. The Marco Polo Wonder was designed as a substitute for this effect.