A port city should have increased trade if it is other trade cities anywhere near water, however you got the trading cities located around, those should also have increased trading. I really have no idea what i am saying but i hope some of it makes sense. if harbors have increased trade then the trading focused land based ones should also have increased trade. However the sea route is more dangerous then the land at some points and other it is not...
One day, someone showed me a glass of water that was half full. And he said, "Is it half full or half empty?" So I drank the water. No more problem. ~Alexander Jodorowsky