The geography of Greece shaped Greek life. As a peninsula surrounded by water on three sides, Greece had many natural harbors. It was difficult to travel overland due to mountains, but the sea allowed Greeks to become skilled sailors and traders who established colonies around the Mediterranean. Two early civilizations, the Minoans and Mycenaeans, influenced Greek culture. The Minoans traded extensively by sea from their base on Crete, while the Mycenaeans built fortresses on the mainland and aggressively expanded their territory through attacks. Both civilizations declined around 1200 BC, plunging Greece into a dark age.