The cities of your race are key to your civilization. Indeed, the very root of our English word "civilization" is the Latin word for "city." Your cities are where your citizens ("city people") live, work, produce, trade, and support your nation. Cities contain buildings which improve the city's productivity and morale, and produce mobile units to project your influence across the globe.
The size of a city is measured in units called citizens. A city's size is visible as a number of the main map, and as a series of citizen icons in the city window. When you create a city with a Settler or Engineer, or discover one from a Village, it consists of one citizen. As the city grows (that is, as the number of citizens increases), more of the land around the city can be worked, and more special talents may be employed.
Each citizen requires two food units per turn to survive; in addition, Settler and Engineer units supported by the city consume food. Each turn, the city produces food from its surrounding land. Any amount of food in excess of that needed for immediate consumption is added to the city's food storage (called the "granary" in the window, whether or not the city has a Granary building). When the amount of stored food reaches the size of the food storage, a new citizen is created, and the food storage is emptied (or, if the city has a Granary, it is reduced to half full). If not enough food is produced in a turn to meet demand, the required food is removed from the food storage; if the store is empty, Settlers, Engineers, or at last resort a citizen are eliminated by starvation.
The citizens of a city may be either workers or specialists. Workers work the land around the city, producing food, production, and trade, as described in Working the Land and the Ocean. Specialists are citizens who are in the city but not working the land. You designate citizens as specialists by moving them off of the map window (see next section); you assign them into one of three specializations:
Each worker citizen is in one of three states of morale: happy, content, and unhappy; the citizen icons in the city window are different for each state. If, at the time of the turn update, the number of unhappy citizens in a city exceeds the number of happy citizens, the city falls into disorder. Cities in disorder produce neither production points nor trade, and are more likely to revolt if prompted by enemy Diplomats and Spies. Prolonged disorder under Democracy can lead to a national revolution; see Democracy.
If the number of happy citizens is at least as great as the number of content citizens, and there are no unhappy citizens, and the city is of at least size 5, the city achieves "rapture." Under the Republic or Democracy, a raptured city increases its population by one each turn as long as there is sufficient food and the criteria for rapture continue.
The normal state of a citizen is contentment. However, as your cities grow larger, crowding causes citizens to become unhappy. By default, each citizen in a city beyond the fourth will be generated unhappy instead of content. If you have a large number of cities (depending on your government), the limit on content citizens is reduced to 3. Under certain forms of government, the presence of military units in a city can either create unhappiness or contentment (see Government Types).
In order to offset unhappiness, you may build buildings (Temple, Colosseum, Cathedral), Wonders of the World (Michaelangelo's Chapel), more make entertainers, or produce more luxuries by setting your national luxury rate. For every 2 points of luxuries produced in a city, one content citizen there is made happy, or (if there are no content citizens) one unhappy citizen is made content.
Large cities, especially in the age of the internal combustion engine, produce pollution. Every turn the pollution of each city is evaluated. There are two sorts of pollution, industrial and population. Industrial pollution is measured by the production points generated by the city, including all improvements such as Factory, possibly reduced by a Recycling Center, Hydro Plant, etc. Population is measured by the city size, scaled up in the presence of Industrialization, Automobile, Mass Production, or Plastics, or eliminated by a Mass Transit system. If the sum of these two types of pollution exceeds 20, that excess is the percentage chance of pollution appearing in a square in the area of a city. This pollution percentage is shown in the city window.
If a city square becomes polluted, it is marked with a special symbol. Polluted squares yield half the food, production points, and trade points. This will continue to be in effect until the pollution is cleaned up by a Settler or Engineer. Further, every turn in which there are polluted squares anywhere in the world increases the chance of global warming.
Global warming is a catastrophic event that models a sudden increase in the size of the oceans as polar icecaps melt. Some number of randomly selected land squares are changed: Coastal Forests become Jungles, coastal Deserts, Plains, and Grasslands become Swamps. Inland Plains, Grassland, and Forest squares become Deserts. An onset of global warming "resets the count" of global pollution, so future pollution contributes to another possible wave of global warming.