In the following screenshots, you can see: An impressive navy built by the Maya in Zama. It's a pity that icebreakers do not exist in this game . You can see the top of the lighthouse emerging from the ice... (Civ4ScreenShot0000.JPG) In Borazjan you can build the harbor even when its path to the ocean is blocked by ice. I was warned for centuries of the presence of that barbarian galley, until I finally researched the tecnology to built a sub and wiped it (Civ4ScreenShot0001.JPG) It seems the AI is treating ice tiles as normal water tiles in its build orders for coastal cities. In the first screenshot, only a fort is needed to obtain the access to the ocean, but the AI seems not to be aware of it... On the other hand, you can't build harbors and lighthouses in cities with access to giant lakes. Even if only a fort or a city placed in the correct spot would grant access to the ocean. The unique exception to this rule is a giant like separated from the ocean by ice tiles.