Greek cities, known as poleis, were independent city-states that emerged in ancient Greece. Each city had its own government, laws, and customs, often centered around a fortified area called the acropolis. The most famous poleis include Athens, known for its democracy and culture, and Sparta, recognized for its military prowess. These cities were often connected through trade and shared religious practices, but they also competed with one another. The Delian League and the Peloponnesian League were alliances formed by various poleis for mutual defense and economic support. This unique structure contributed to the rich history and development of ancient Greek civilization.