In my current game the world is split into three continents (henceforth C1, C2, C3). I started off in C1 and eventually got all my cities linked to my capital. Later on, to assist my conquest of C2, I built a coastal city ("Venture") and placed a harbor in it for instant trade route link. I conquered Arabia and Egypt, razing everything but Mecca and Thebes. Over the next 1000 these cities flourished and I decided to link them to my trade network.
I assumed this would be as easy as linking them via road to Venture, since Venture was connected to the trade-net via the harbor. Yet neither Thebes nor Mecca don't have the trade icon beneath their names, nor have I received notice that they are now part of the trade route.
Is my assumption wrong? How is one supposed to get landlocked cities on other continents into the trade network?
I assumed this would be as easy as linking them via road to Venture, since Venture was connected to the trade-net via the harbor. Yet neither Thebes nor Mecca don't have the trade icon beneath their names, nor have I received notice that they are now part of the trade route.
Is my assumption wrong? How is one supposed to get landlocked cities on other continents into the trade network?