You need a connection between your capitol and the capitol of the civ you want a trade route with. This can be done with any combination of roads, harbors, and airports.
If there is any road connection between two cities, including through other cities, they are trade connected; so all you'll need to connect yourself to a civ whose capitol is also on the same continent is roads.
A harbor can connect with any other harbor over any water tiles you have the ability to cross - coast from the beginning of the game, sea with Astronomy, and Ocean with either Navigation or Magnetism. Note that harbors can be blockaded from accessing another harbor if units of a civ you are at war with block all possible paths to the other harbor. However, as the ocean is quite wide and vast, pretty much the only way for a civ to blockade a harbor of yours is to have units on the coast tiles adjacent to your city.
An airport can connect with any other airport on the map.
Again, any combination can be used to reach the other civ's capitol. You could have your capitol connected by road to a city with a harbor, with that city's harbor connecting with the harbor in another of your cities on another island that has an airport, and with that city's airport connecting with one of the other civ's cities that has an airport, and with that city connecting with that civ's capitol. Harbors and later Airports are generally in enough abundance that you don't need as long and complicated a chain as that, but it would work.
If there is any road connection between two cities, including through other cities, they are trade connected; so all you'll need to connect yourself to a civ whose capitol is also on the same continent is roads.
A harbor can connect with any other harbor over any water tiles you have the ability to cross - coast from the beginning of the game, sea with Astronomy, and Ocean with either Navigation or Magnetism. Note that harbors can be blockaded from accessing another harbor if units of a civ you are at war with block all possible paths to the other harbor. However, as the ocean is quite wide and vast, pretty much the only way for a civ to blockade a harbor of yours is to have units on the coast tiles adjacent to your city.
An airport can connect with any other airport on the map.
Again, any combination can be used to reach the other civ's capitol. You could have your capitol connected by road to a city with a harbor, with that city's harbor connecting with the harbor in another of your cities on another island that has an airport, and with that city's airport connecting with one of the other civ's cities that has an airport, and with that city connecting with that civ's capitol. Harbors and later Airports are generally in enough abundance that you don't need as long and complicated a chain as that, but it would work.
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