Ports are buildings that can be constructed after you have researched the Sailing technology. Ports produce 2 population to the city it is built in. They cost 10 stars and can only be built on water tiles that are in your territory.
Sea Routes Edit
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 and give +1 population to the connected city and the capital, functioning like roads. However, sea routes do have a tile limit, introduced in the "Power" update. Although this tile limit's exact quantity has not been found, it has been confirmed through many games that there indeed is a limit.