Ancient Greece was never a unified country, but was divided into independent city-states called poleis. Two important poleis were Athens and Sparta, which fought against each other in the Peloponnesian Wars in the 5th century BC. Alexander the Great later conquered the Persian Empire and much of the known world in the 4th century BC, spreading Greek culture and establishing Hellenistic kingdoms. Greek society was unequal, with citizens, foreigners, and slaves, and their economy relied on agriculture, trade, and crafts. Greeks made advances in architecture, following three orders, and sculpture, becoming more realistic over time.