Cities can have trade routes between them. Every turn that a trade route exists between two cities, each city receives a number of trade points based on the current trade of each city; if you make a trade route to another civilization's city, both of your civilizations get increased trade. A trade route is established by Caravan or Freight units. A city produces such units as it does any unit. After production, the player moves the unit to another city, of the same nation or another. The best returns come from trade routes to distant cities, on other continents if possible, in other civilizations if possible. When such a unit enters another city (at least eight squares from its home), the player is asked if a trade route should be established. If so, the unit is destroyed, and a trade route now exists. The owning player gets an immediate bonus in the creation of the trade route.
If a city of yours is captured by another civilization, there is a chance that it will spawn Partisans who are owned by you, if all of the following are true: