I just finished up WOTM 06 as ragnar with literally every city on the coast. The trade was huge and probably was at least 40% of my economy. I then played a game as carthage on another map and I built my empire inwards making lots of commerce cities not on the coast. Even though I had far more recources this game, and was far more developed than I was as ragnar, my economy was literally 30 - 40% weaker than it was as ragnar. Ive concluded, that every commerce city you make, with very few exceptions, should be built on the coast, and all your industrial cities should be inland.