City connections, also known as trade connections, are created when a player's capital is connected to one or more of their other cities by roads, Ports, Outposts, and/or Mycelium. A city connection grants one population to both the capital and the connected city. A connected city will have a road symbol next to its name.
Building a continuous path from a city to the capital or an already-connected city with any combination of roads, Ports, and Outposts will complete a city connection. City connections can use roads in neutral territory but not those in enemy territory.
Ports and Outposts can form connections with other Ports or Outposts that have three or fewer water or ice tiles between them. Therefore, roads are not required to form city connections if there is a Port or Outpost adjacent to the capital. (However, researching the Roads technology is required to unlock the Network task.)
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, e.g., no one can break a city connection by connecting their city to a piece of neutral road used to form a city connection.
Connections can be broken if a connected city 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 population provided by the city connections and may lead to negative population (especially in the capital).
- Before the "Power" update, connections over water were not limited in length.