Which geographic feature served as a barrier to political unity and encouraged the rise of independent city-state in ancient Greece?

The mountainous geography of ancient Greece served as a significant barrier to political unity, as the rugged terrain limited communication and transportation between different regions. This fragmentation led to the development of independent city-states, or poleis, each with its own government and customs. The natural divisions created by mountains and valleys fostered local identities and rivalries, making it challenging for any single political entity to dominate the entire region. As a result, city-states like Athens and Sparta emerged, each thriving in relative isolation.