What factors united the city-states of Greece?

Greek city-states likely developed because of the physical geography of the Mediterranean region. The landscape features rocky, mountainous land and many islands. These physical barriers caused population centers to be relatively isolated from each other. The sea was often the easiest way to move from place to place. Another reason city-states formed, rather than a central, all-encompassing monarchy, was that the Greek aristocracy strove to maintain their city-states’ independence and to unseat any potential tyrants.