Ports are buildings that can be constructed after you have researched the Sailing technology. They cost 10 stars and can only be built on water tiles that are in your territory. Ports produce 2 population to the city its built in. When you have 2 ports or more, a sea route will form as long as it won't be blocked by land tiles, similar to roads. Sea routes will connect cities along with roads. Each city connected to your capital gives it 1 population.