.jpg){kind=link}
London, a city of England in Civilization V
A city is the basic unit of a civilization, and in many ways they are its most valuable type of possession. Cities do most of the advancement in a civilization, generating 
Food Food, 
Production Production, 
Gold Gold, 
Science Science and 
Culture Culture, as well as 
Faith Faith (in Gods & Kings) and 
Tourism Tourism (in Brave New World) for an empire. Cities also have the ability to produce buildings and units, and build Wonders. All tiles with cities on them have a road, even if The Wheel hasn't been discovered yet.
A civilization is eliminated from the game (unless the "Require Complete Kills" option in the Custom Game menu has been selected) when all of its cities have been captured.
In the beginning of all games, scenarios and mods, civilizations will either have cities laid out or start with at least one Settler to build a city. In advanced start, cities must be built before any units may be placed on the map. For more information on the effects of settling a city and tips of successful settling, check this article.
Border Expansion
An empire's borders are created and expanded by cities. Each time a new city is founded (starting with the 
CapitalCapital), an empire's territory expands into the tiles immediately adjacent to the new city. For more information on this, refer to the Territory article.
City Population and Work
Cities are locations where an empire's people are clustered. In the game rules and mechanics, a city has a certain Population, which corresponds to its number of 
Population Citizens. Population in a city starts at 1, and will continue to grow under the right circumstances. Besides determining the size of the city, citizens serve as the workforce of an empire. They are assigned to "work" the land around their city (again, up to three tiles away), or fill the specialist slots created by certain buildings inside the city. This assignment is automatic, but can be reassigned manually from the city screen.
Without citizens working the land, the surrounding terrain's production potential (which in this case means all possible types of generation, such as food, production, gold, etc.) remains unused, and will not produce any of the products available on any given tile. When a tile is worked by a citizen from a nearby city, its production potential is added to the city's total production. The tile the city is located on is automatically worked for free.
Generating yields
The overall yields of a city, or how much exactly of each statistical resource it is producing, depends on many factors, but paramount among them are:
- Population (in
Population Citizens), and their distribution (which tiles or Specialist slots) to where they are assigned to work. Tile yields are affected primarily by Terrain, providing the "base yield". This is the primary method of aquiring Food and Production. Prior to Brave New World and trade units, this may also include Gold Gold. - Buildings (including Wonders) constructed in the city, which have with more varied yields and other bonuses, as well as providing the aforementioned specialist slots. This is also the main method of acquiring Culture Culture for border expansion. City or empire modifiers (such as the National College 50% bonus to Science yield) also add to the base amount for each resource, as do temporary bonuses. The upper left corner of the city screen shows how much of each resource the city is producing.
Depending on what resources the empire, or that particular city, needs, consider carefully where to assign citizens and also consider what improvements should be constructed on the surrounding lands to improve upon the base yield (especially Farms to increase FoodGrowth and increased population); additionally, always have a strategy for the long-term development of a city, and use it to determine what buildings to build in it, and in what order.
Population Growth
The population growth of a city, or the birth of new citizens, is determined by the city's available Food Food. The more Food Food a city is producing, the faster its population will grow. For more details about Food Food and Population Population growth, check this article. Population growth will slow dramatically if an empire is not 
Happiness Happy.
Producing items
Every city has the ability to produce buildings, units, or projects. The availability of each depends on whether the technology unlocking it has been researched, and whether its prerequisites have been met. For example, certain buildings may only be built if a city is in a tile adjacent to a coast, or if a certain type of building has already been built in all of a civilization's cities, etc. Once all prerequisites have been met, the building or unit can be assigned as the next production project.
Everything a city can produce has a total Production Production cost, measured in Production Points (PPs), which must be reached in order to complete the item. The number of turns it takes to complete production is determined by the total amount of PPs produced by the producing city per turn. When a city has spent enough turns and generated enough production towards the item in question, and when the total is reached, the project is done. As a rule, the more advanced the thing being built is, the more Production Production it costs, so as the eras progress, a city either needs to increase its Production Production, or take more turns to produce what it needs.
For more information on Production Production, check here.
Buildings
These are permanent structures erected in a city. Once built, they produce their benefit continuously. Only one of each building may be built in each city.
Almost all buildings cost an empire some Gold Gold as maintenance each turn. Gold-generating buildings (such as the Market) and defensive buildings (such as the Castle) have no maintenance.
For more information on buildings, Wonders, their qualities and effects, check here.
Units
Both combat and non-combat units may be produced in cities. Unlike buildings, an empire can produce as many units as desired (assuming the required resource, such as Iron or Oil, is available), but each unit costs Gold Gold to maintain. Also, there is a limit to the total number of units an empire can build without penalty. This is based on population; for example, an empire with a population of 8 may have up to 17 units (including Workers). For each unit over that, production empire-wide is reduced 10%. In this case, a red dot with a white, angled exclamation point will appear at the top of the map to the right of the strategic resources icons. Clicking on the icon will reveal the percent reduction. If Raging Barbarians is active, this can easily happen to Germany if it recruits too many. Once finished, units appear in the city they were produced in, ready to be moved out and used.
Production of a certain unit or building can be stopped and resumed as many times as desired until it is finished. In the case of Wonders, each city can start the project (if it meets the necessary prerequisites), but production of the same project cannot be transferred between cities. For example, a civilization can't start the Colossus in City A, then stop it, and continue it in City B from the same production point. If another civilization completes building a Wonder first, then it will be unable to be completed and the accumulated Production Production will be converted to Gold Gold.
Gold purchasing
Gold can be used to instantly purchase any building or unit which has been unlocked empire-wide in a city (in the case of a building, the city also must fulfill the necessary prerequisites). If there already is a unit in the city in which a purchase is being made, it may need to be moved out of the city before purchasing another unit in accordance with the rules concerning unit stacking.
In the Gods & Kings expansion, certain units and buildings can be purchased with Faith Faith. See Religion for more info.
Capturing Cities
During war, a city may be captured if a city loses all its health points and an enemy melee unit manages to move into it. The city loses a portion of its population in the process (roughly half of it), and it takes some turns for all damage to infrastructure to be repaired and for the city to return to full health. Also, all defensive structures (Walls, Castles, Arsenals, and Military Bases, or their replacements), and a random selection of other buildings in the captured city are destroyed.
Capturing a city gives the following options:
- Annex the city - This means taking the city and officially making it part of the empire. That gives full control of it, meaning build units may be built, workers assigned, and so on in the city. The city is, however, counted as "Occupied," and adds extra Unhappiness Unhappiness that can only be eliminated by building a Courthouse in the city.
- Make the city a puppet - Install a puppet government, declaring that the city is "autonomous" and not really part of the empire, even after the capture. Puppet cities add their territory to the empire, and contribute their resource production to the empire's Culture Culture, Science Science, etc. Workers cannot be assigned or city production directed, and it is not possible to buy units or buildings in the city except for in puppets owned by Venice. Relatively less extra Unhappiness Unhappiness is produced from the city than an Occupied city, and that the city doesn't count in an empire for purposes of Social Policies and Golden Ages. Puppet cities may be annexed at any time.
- Raze the city - Causes the city to be destroyed. The city's Population Population is reduced by one per turn, until it reaches zero. At that point the tile the city was on gets a City Ruins and a pillaged road, and the territory controlled by it vanishes. Beware though, that an empire's Unhappiness Unhappiness will spike sharply when razing a city, then decline rapidly as the city burns down.
- Liberate the city and give it back to its original owner - If the city belonged to a third party or was an independent city-state before being captured, it can be to returned it back to its former owner, thereby placing the city under its previous owner's control. This can bring back eliminated civilizations and free city-states. This option nets a large diplomatic bonus with the liberated city's civilization or results in a city-state becoming an Ally.
Cities can also be "traded," but this only happens under certain circumstances, almost always in the case of peace offerings from the losing side of a war. Traded cities are treated as if they were obtained through conquest (save for Assyria's unique ability not getting activated). For a more detailed info on conquered cities, read this article.
Resistance
When a city is acquired, be it via conquest or diplomacy, its population will resist the new owner's domination for a number of turns, usually equal to the remaining population, less if the population is very large. While in resistance, the city cannot build anything, and do not contribute to a civilization's resources. However, it immediately adds to the empire's 
Unhappiness Unhappiness. If an empire completes the Wonder CN Tower, which adds population and a Broadcast Tower to every city in that empire, the new tower will appear in cities still in resistance.