Ancient Greece was located in the Mediterranean region known as the Aegean Sea, this civilization was formed by 4 different geographic areas: 1) Peloponnese, this area was located in the southwestern peninsula and it only had a small part of land that connected it to the mainland, a famous city-state of this region is Sparta. 2) Central Greece, this area was above the Peloponnese, a famous city-State from this region is Athens. 3) Northern Greece, this big area grew -towards northeastern land thanks to conquest campaigns and was divided in 3 different sections: Thessaly, Epirus, and Macedonia. 4) A numerous groups of islands located in the Aegean Sea formed the last geographic area.
As the question says, the mountainous territory served as protections against invaders, but it also meant it was not easy to travel by land between regions, and that each city-State formed a particular way of life and culture. The regionalism in ancient Greece also implied a constant internal conflict between city-states like Sparta and Athens, and the inability of having a central government because of the big differences and conflicts between population.
