I have the following set-up:
- My Capital (C) has a road to a coastal city (A)
- My coastal city has a harbor
- Another "coastal" city (B) is on an in-land lake, and has a harbor
- Another coastal city (D) has a harbor
- I haven't lifted enough fog of war to show a water connection between city D and city B.
- city A has a trade route, but city B and D do not.
- I'm playing as the Iroquois, so some of my "road" is forest tiles in my territory.
Here is my setup in ASCII art.
~ ~ ~ D______ _____________ __~ ~ ~__C___ _B~ ~ ~__|___ __~ ~ ~__A~ ~ __________~ ~ ~ = water _ = land | = road A, B, D = cities C = capital
Why isn't D connected to the trade route? Do I need to scout a path of water tiles between its harbor and A's for the trade route? Do "lake" harbors never connect to "sea" harbors or could scouting a path of water tiles/river tiles connect the two?
More broadly, what are the requirements for harbors to connect to your trade route?