Thebes was created as a trading post between the two halves of the northern continent. Even with the tribes occupying the grasslands it was easier to carry goods by ship to Thebes and then, by caravan to Magdan then crossing the mountains.

There are, of course, other passages north, mostly in the vicinity of the internal sea controlled by the Romanish empire. Rivers flow to that sea, and some of them almost cross the continent. Almost is the key word here, the whole continent is divided in two by the mountains and they get bigger as you go north so that a distance that looks small in a map proves to be full of ups and downs in real life.