It is the period of classical Greek civilization that followed the so-called Dark Ages of Greece, which came to an end about 800 B.C. Over time the Dorians had become more settled, and they gradually revived trade and culture on mainland Greece. The self-governing city-state (polis) evolved, including the military center of Sparta and Athens, which became a center for the arts, education, and democracy. This was the beginning of the great Hellenic period of classical Greek civilization. Greek civilization reached its height in Athens during the mid-400s B.C., a period of outstanding achievement known as the Golden Age.