What are the requirements for harbors to connect to your trade route?

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?

Solution 1:

The requirements to connect two cities can be summed up in one sentence plus some caveats: there must be an explored path between the two cities, made of friendly or neutral territory.

The caveats:

1. "Friendly or neutral territory" is owned by you, owned by a friendly (or allied) City-State, owned by a civilization you have Open Borders with, or unowned.
2. The path between two cities can be over land or over sea. There's no such thing as a single path between two cities that involves both land and sea.
3. Land routes require a road along the entire path.
4. Sea routes require a Harbor at each end.
5. A city is connected to the capitol if it is connected to any other city that is connected to the capitol. (This is recursive; you can have a line of cities, each connected only to the city on its left and the city on its right, and all will be connected to the capitol and give you the city connection money. Each connection can be either by land or by sea, as long as each individual connection is entirely land or entirely sea.)

So when we apply those rules to your diagram, here are the connections:

1. A is connected to C by a road.
2. B is not connected to anything; because it doesn't have any roads, the only paths possible from B are water paths, and there is no other city on that inland lake to connect to.
3. D could be connected to A if you had explored a path all the way around the continent, and the path wasn't blocked by unfriendly City-States or civilizations. (Before you get caravels, it's common for long sea paths to be blocked by at least one other civilization or City-State, possibly several. See this question for a path blocked by three City-States plus one civilization.) Once you have Caravels and have explored the deep oceans, it'll be much easier to have an explored route that isn't blocked by someone unfriendly.