I cant see much importance in trade for the Twins crossing cause the area between green fork and blue fork seems a lot populous and fertile, as seem the area around Seagard. But the area between green fork and the moon mountains doesnt seem to have any town or keep. Also i dont believe there is much trade heading north from riverlands into the neck. So, is it believable that the Freys got so rich? For military reasons the Twins are a must have, but for trade i cant see why.