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
It is unclear to me though whether I need a harbor in the capital to make the trade routes from other harbors work … ?
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 |~ ~ ~ ~ ~ ~|
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
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
3 were on the same continent, but not connected by road/rail.