Greece is located in Southeastern Europe, between the Ionian and Aegean Seas. The ancient Greek civilization developed across mountainous terrain and along coastlines. Greece comprised independent city-states called poleis that never formed a unified state. During the Archaic period, the Greeks established colonies across the Mediterranean and Black Seas. The Classical period saw the Greco-Persian wars and Peloponnesian wars between Athens and Sparta. Alexander the Great later conquered the Persian Empire and expanded Greek rule throughout Europe and Asia. Greek culture flourished through advances in architecture, art, philosophy, and the establishment of democracy in some poleis.