The manual (and the civilopedia) state for the harbor:
Effects: Forms a naval Trade Route
with the Capital City, +25% Production
of Naval Units, City must be on a
coast.
It is unclear to me though whether I need a harbor in the capital to make the trade routes from other harbors work … ?
Best Answer
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:
City
1
is your capital, and linked by road to city2
. If city2
and city3
both have a harbour then they extend the trade route across the water, this means4
and5
are also connected, because they are linked to3
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 in3
alone is not sufficent.This also works if
2
and3
were on the same continent, but not connected by road/rail.