Cities may be enhanced with buildings, each with a different effect. Some buildings require others. Most buildings become available when you achieve certain technologies, while technology makes others become obsolete.
It costs production points to construct buildings — often taking several turns. Once completed, many buildings require an upkeep of gold. You may dismantle and sell a building, receiving one gold for each production point used in its construction. If a turn comes where you cannot pay the upkeep on all your buildings, some will be automatically sold. Obviously this should be avoided.
Wonders are unique structures that confer special advantages to your civilization. While buildings affect only their own city, many wonders benefit all your cities on the same continent. Caravans and Freight built in other cities can contribute their full cost in production points towards the construction of a wonder.
Citizens are usually workers who extract resources from one terrain tile. But there are three specialist roles citizens may assume:
An entertainer produces two luxury points for their city. A tax collector provides three extra gold per turn for your treasury. A scientist adds three points to your research output.
When your cities grow and produce new citizens, the game starts them off as workers. The game assigns new workers to the terrain. You will want to inspect cities that have just grown and adjust the tile or role for which the new citizen has been placed.
Three buildings allow you to multiply the effects of scientists and total research produced by your city: Library, University and Research Lab
Other buildings multiply the gold, luxury, and effects of your entertainers and tax collectors: Marketplace, Bank and Stock Exchange
Unfortunately, city growth produces crowding which makes it difficult to maintain worker morale. Each citizen is either happy, content, unhappy or angry. Only the first four workers are naturally content; the rest are naturally unhappy, which is quite serious as even one unhappy worker can throw the city into disorder. Cities in disorder produce no food or production surplus, science, or taxes; only luxury production remains. They are also more prone to revolt, and prolonged disorder in a democracy can even result in national revolution.
Only workers vary in morale. Entertainers, scientists, and tax collectors enjoy enough privilege to remain perpetually content. Thus, one solution to the problem of an unhappy worker is simply to assign that citizen to the role of a specialist. But if cities are ever to work more than four terrain tiles at once, the problem of morale must be solved.
There are two means of saving large cities from disorder. Most of the buildings and wonders that affect morale make unhappy workers content, which prevents disorder. The more interesting option is to produce happy workers. These can balance the effect of unhappy workers — a city will not fall into disorder unless unhappy workers outnumber happy ones. Happy workers can also produce other desirable effects.
Cities with three or more citizens celebrate when half or more of their citizens are happy workers and none is unhappy. Under Monarchy, Communism, and Fundamentalism, this gives a +1 trade bonus to trade tiles around the city. Under Republic or Democracy, a city that was celebrating on the turn before will enter rapture and grow by one citizen each turn that the city has a food surplus. Without rapture, large cities grow very slowly, waiting dozens of turns for their granary to fill.
Managing morale or happiness and balancing it with taxation rates is the key engine of economic management in the game. Workers are made happy when you provide them with luxury. For every two luxury points a city produces, one content worker is made happy (or if there are no content workers left, one unhappy worker becomes content). Besides the luxury points produced by entertainers, cities receive some of the trade points they produce as luxury points, proportional to the nationwide luxury rate you have set.
Military units can affect city happiness. Under authoritarian regimes this is helpful, as military units stationed in a city can prevent unhappiness through martial law. Under representative governments the only effect is negative — citizens become unhappy when their city is supporting military units which have been deployed into an "aggressive stance." This includes all units not inside your borders, an allied city, or a fortress within three tiles of a friendly city. Field units (missiles, helicopters, and bombers) cause unhappiness regardless of location. See the section on governments for the number of citizens affected by each of these factors.
All of the above discussion assumed that cities can grow to size four without unhappiness, with the fifth citizen being the first unhappy. This limit of 4 citizens decreases as you gain more cities, simulating the difficulty of imposing order upon a large empire. Different governments can support different numbers of cities before encountering this limit for the first time; see the section on government for details.
In empires that grow beyond the point where no citizens are naturally content, angry citizens will appear. Angry citizens must first be converted to unhappy before they can be made content. In all other respects, angry citizens behave as unhappy citizens.
Temples, Courthouses, Amphitheaters, Cathedrals, Marketplaces, Banks, Stock Exchanges, and Police Stations can all influence happiness in different ways.
The Wonders which affect happiness in different ways are the Oracle, Hanging Gardens, Great Library, Marco Polo's Embassy, Michelangelo's Chapel, Shakespeare's Theatre, J.S. Bach's Cathedral, Women's Suffrage, and the Cure for Cancer.
Cities may not grow beyond size 8 without an Aqueduct, and may not grow beyond size 12 without a Sewer System.
The buildings that affect city growth are: Aqueduct and Sewer System.
The buildings that affect worker happiness are: Temple, Courthouse, Amphitheater, Cathedral, Police Station.
Pollution can afflict large cities when your civilization becomes more industrialized. The chance of pollution appearing around a city depends on its total production output. When this sum exceeds 20, the excess is the percent chance of pollution appearing each turn. The technologies industrialization, mass production, automobile, and plastics, each add 25% of your total population to the foregoing sum. Thus, a size-20 city with 18 production and one of these technologies, would arrive at 18+(0.25*20)=23, giving it a 3% chance of pollution. The pollution percentage is shown in the city dialogue of the city window.
Pollution appears as gunk on terrain tiles around the city. The pollution can only be cleared by dispatching workers, settlers or engineers with the clean pollution order (which takes 2 worker-turns to complete.) Polluted terrain generates only half its usual food, production, and trade.
When an unused tile becomes polluted, there is the temptation to avoid the effort of cleaning it; but the spread of pollution has terrible results — every polluted tile increases the chance of global warming. Each time global warming advances, the entire world loses coastal land for jungles and swamps, and inland tiles are lost to desert. This tends to devastate cities and leads to global impoverishment.
The Environmentalism technology reduces pollution. Several buildings also affect pollution: Hydro Plant, Mass Transit, Recycling Center, Nuclear Plant, Mfg. Plant, Solar Plant.