Bug: Trade Routes
I've noticed, that if for some reason you have researched the compass, and build a harbor in both your capital and another city you won't establish a trade route unless you have also researched the wheel.
It says nothing of that sort in the tech description.
This is probably a rare issue since the compass is a medieval tech and the wheel is usually discovered in the ancient era, but still it seems to be a bug.
You need the wheel to build the 'road' in the city that activates traderoutes. same with iroquois bonus if you want to set up trade routes. Simple fix, in wheel description it should say need trade route.