I could see some specific uses for harbors in corrupt cities, but in general, there is no need to build a harbor to reach 5-6 citizen, especially if you are irrigating and railing everything. If you want to build something, why not build workers, settlers or artillary? The issue I have is that most cities in the time of rails will naturally hover around 5-6 citizens with irrigation and rails. You can support 3-4 taxmen per city. You put it on wealth and you receive an extra 1gpt. You build a harbor. That is -1gpt. If that allows the city to grow past 5-6 citizens without the need to build and aqueduct, then it might be worth it. If you are supporting an aqueduct to enable the growth, that is another -1gpt. I see it as ROI. If you need some combination of improvements to support the larger population of taxmen, then you need to make sure you will recoup the cost. You also need to compare. The harbor costs 40-80 shields. Average is 60 shields. So for the same price, I can make two settlers. Those two settlers can run off and make two new cities, each set to wealth and supporting 3-4 taxmen each. So, for the same investment, I can create +8-10gpt. It would be pretty hard for a harbor to compete with that. Better yet, set those cities to +6-8gpt and build four more settlers. Now if you have already packed all of your land with as many settlers as it can hold and you are still looking for a way to pick up a few extra gpt, then I suppose, what the heck do you have to lose? Harbors in C3C might be a different argument, though. I don't play C3C so I cannot comment.