Send boats from one island to another in order to conquer them! The population of captured islands increases over time, and some islands produce gold, which allows you to upgrade them. The population of an island determines how many boats it can send out and how much gold it produces.
There are several type of islands, and each could be used for its own purpose:
Colonies produce more boats. Can be little, middle or huge.
Mine produces gold for upgrades. The more population it has, the faster gold it produces. The more gold you have the more islands you could enhance and the more boats you could produce.
Tower protects everything near them by shooting at enemy ships. Shooting speed determined by level, gun power depends on tower population.
Shipyard produces fast boats, perfect as anti-tower unit.