City connections, also known as trade connections, are created when a city is connected to your capital by Roads, Ports, or Outposts. They grant one Population to both the capital city and the connected city.
Building a continuous path with any combination of Roads, Ports, and Outposts from a city to your capital (or to an already-connected city) will complete a city connection. You may form city connections with Roads in neutral territory (but not Roads in enemy territory).
Ports can form connections with other Ports that have three or fewer water tiles between them. (The same applies to Outposts, but they can only form connections over Ice tiles.) Therefore, buying the Roads technology is not required to form city connections if there is a Port or Outpost adjacent to your capital (although the Roads technology is required to get the Network task). Naval trade routes are often buggy; for example, if the Ports form a straight diagonal line with three water tiles between them, they may not connect, and sometimes a naval trade route is established between Ports with four water tiles in between.
In multiplayer games, if a piece of neutral Road is connected to more than one player's city, only the player that comes first in host order (listed first in the multiplayer games menu) can use that piece of neutral Road to form city connections. (This does not impact established city connections, i.e., no one can cause you to lose a city connection by connecting their city to a piece of neutral Road you used to form a city connection.)
Connections can be broken if a connected city or capital is captured. If a city is captured, then all connections using the Roads in that city's territory will also be severed. This will remove the one bonus population provided by the city connection for each connected city, and may lead to negative population (especially in the capital).
- Before the "Power" update, connections over water were not limited in length.