City Links are the connections of your city to the outside world, or the rest of the Planet if you prefer. They are essential for your economy, because they not only allow external trade, but also provide Freight and Passenger Services for your city.
City Links depend on map resources - each map's geographical features allow different posssibilities for connection. Some maps have relatively more connection resources, while others are pretty limited in that direction, and this also makes part in determining a map's difficulty.
Landlocked maps provide plenty of road connection resources, but no sea connection; island provide sea connection, but no road connection, etc. All maps allow air connection, but airports only provide passenger transport.
To see each maps's specific connection resources, open the Resources Layer - there you'll find all four types currently available in the game (three, actually, because air connection is possible everywhere there is sufficient land to plop the airport). The road resource areas appear as a purple aura along landmass borders, while sea resources appear white along coasts.
Each connection that you make will use up part of the resource area (which will simply vanish, making it impossible for you to make two connections next to each other). This limits the practical number of connections you can make with each type, so be careful and plan ahead! Also, each connection needs monthly maintenance, which is separate from the maintenance fee of the infrastructure, used to make the connection.
As mentioned already, all City Links provide trading capacity for your city. This is measured in units of Freight and Passenger capacity, and each unit allows trading of 1 token of a resource, using the respective means of transport (Freight or Passengers).
You will start the game with at least 20 capacity for both, since the first thing you do in every new city is to place a City Link. As the city grows and its trading needs increase, you'll need to make many new connections - sufficient to allow for your city's goods movement.
To check current trading capacities, open the Resources Panel>Transportation subpanel. Here you'll see total capacity, and also capacities broken down by transport type.
Types of connections
There are several means to build City Links. Each one uses a different map resource (that is, it acts separately from the others), but all of them provide both external trading capacity and Freight and/or Passenger services.
This will be your basic connection for landlocked maps. It uses the Road resource on Land. You make this connection by using any normal road in the game (up to Expressway). Trade capacities for each City Link (regardless of the type of the road used) is 20 Freight, 20 Passengers. Note that this is the cheapest and most accessible connection type, but it provides the least trading capacity.
This connection is done via Highways and uses the Highway resource on Land. It is the most expensive connection by far, costing 50 000 c/mo, but it provides 100 units of Freight and Passengers. Still, it's not cost efficient, so use with caution only when you have run out of other options.
This is your basic connection on island maps. It is done via Harbors. Each harbor constructed automatically makes a sea connection, providing Freight transport, or if you're on island map - both Freight and Passenger transport. There are currently two types of harbors in the game - Small and Large harbor, although upcoming mods promise to change that. The NEXL is making significant changes in the way Sea connections function, and these will be revealed soon as the mod nears completion.
Done via Airports, the air connection is the only one that doesn't use a resource. As long as you have sufficient space in the city, you can build airports and establish new connections. Of course, you'll need to plan very carefully where you'll do that, since airports require a huge expanse of flat land. Airports only provide Passenger services. NEXL promises changes here too, with the introduction of Airways.
The last type of connection, the train connection via Land, was partially coded by Monte Cristo, but never fully implemented. NEXL is working to bringing this vital alternative to Road and Highway connections to life, so we expect its future development!