From ICv2:

The Island, which is also a card game, takes place on an island containing a number of warring city states. Players begin with their own city, which comes complete with an army that can be augmented by mercenaries and also by conquering other cities and absorbing their forces. Some cards also add a magical component that can be used to augment a player’s forces in battle or to strengthen the defenses of a city. The player who conquers the most cities is the winner.