Civilization – Do internal traderoutes drain your host city

The trade network is just that - a network.

In the same way that a city can be linked by road via another city already linked by road - they don't all need their own roads directly to the capital - your trade network will spread via harbours.

What this means is you need at least one harbour within each "sub network" of your trade network. Here is a quick diagram:

Continent A     Water     Continent B
            |~ ~ ~ ~ ~ ~|
     1------2 ~ ~ ~ ~ ~ 3------4-----5
            |~ ~ ~ ~ ~ ~|

City 1 is your capital, and linked by road to city 2. If city 2 and city 3 both have a harbour then they extend the trade route across the water, this means 4 and 5 are also connected, because they are linked to 3 and so, by proxy, use it's harbour link.

If in this example, even if 2 was your capital it would still need a harbour to extend your trade route accross the water, having a harbour in 3 alone is not sufficent.

This also works if 2 and 3 were on the same continent, but not connected by road/rail.

While the caravan/cargo ship does have a graphic that goes back and forth, the trade route gives food/production/gold per turn, not per graphical trip.

I just tested the effects of distance in my current game (all routes comes from the same port city).

Internal cities:

I sent 2 cargo ships to cities in my own empire:

• City 1 is 10 hexes away, the trade route gives 9 production/turn, for 30 turns.
• City 2 is 21 hexes away, the trade route gives 9 production/turn, for 30 turns.

So, distance does not effect how long internal trade routes lasts, or how much food/production you get (from what I can see).

Foreign Cities:

I tested it with some cargo ships to foreign cities and city states:

• City 3 is 28 hexes away (other civ), trade route lasts 28 turns
• City 4 is 29 hexes away (city state), trade route lasts 29 turns
• City 5 is 40 hexes away (city state), trade route lasts 40 turns
• City 6 is 42 hexes away (city state), trade route lasts 42 turns

It does effect the length of time that a foreign trade route remains active, the trade route appears to last 1 turn per hex of the trade route length.

Distance does not appear to influence the calculation of gold/turn with foreign cities. Based on the trade screen, that is related to "difference in resources", and the two cities local gold/turn generation.

Defence:

Distance will be a consideration in the defence of trade routes, because if a trade route is very long you will have more distance to protect from barbarians or enemies.