There are cases when players want to migrate to different continents. For example, if a lot of alliance members are living all around the map, coordinating activities is very challenging. Thus, many advanced alliances require migration to a primary continent.
Cities can be built on other continents. In order to do that, one needs to have a level 10 harbour (or two that provide 25 ships). There is a significant gold bonus for completing the quest of settling multiple continents.
Travel time between continents is long. For example, between continent 55 and 42, travel time is more than 24 hours for one direction. Thus, it will take 48 hours to get ships back to the originating city.
In order for the overseas city to receive goods from sea travel shipments, it must have a Marketplace. A harbour is not needed in the receiving city. This is different from land transport; receiving resource shipments that are sent by land transport do not require a Marketplace. (You now are able to send resources to an overseas city without a Marketplace).
Overseas city does not need to be a coastal city. A tile bordering a river can also be reached by ships (but only straight angles, not diagonal).
If you send a baron to settle a tile, and a new player is placed there before your settlers arrive, he will be sent back to the city of origin with the 250k resources that were sent with him.
Empty continents cannot be settled by existing players until the server opens the continent for new player settlement.