Much of Greece is covered by mountains. This made it hard to travel around by land in ancient times. But Greece is also surrounded by sea on three sides, and has many islands around the mainland. So the sea was always important to the ancient Greeks. They travelled by sea as much as they could. This meant they needed harbours to land in. Some of these harbours grew into ports - places where ships came from all over ancient Greece. Ports were especially useful to traders, because they could meet there to trade.