It immediately expands the city's borders by one tile in every direction if those tiles are not part of another city (either your own city or an enemy tribe's). For this reason, it is sometimes better to save upgrading to level 4 until all the villages around it have been captured by you. It will also explore a 5x5 area around your city, regardless of the tile being in enemy territory or not.
You get +20 points for every new tile in your city.
|Border Growth, City Wall, Explorer, Park, Population, Population Growth, Stars, Super Unit, Workshop|