What are the positive and negative impacts of geography on the development of Greece?

The geography of Greece played a couple of major roles in helping to mold Greek society, particularly during the ancient period. For one, Greece is very mountainous. This can be viewed positively because mountains serve as natural barriers, and thus as a form of protection from outside invasion. The downside of mountains acting as a barrier would be that they also serve as a barrier to trade and cultural diffusion. The mountains of Greece have been cited as one of the reasons many unique and different city-states developed in Ancient Greece.
In addition to mountains, Greece also consists of many islands, primarily in the Aegean Sea. As a result of that, the early inhabitants of Greece had to become well-versed in trading by sea. The peoples of the Greek islands would also utilize their location on the sea to become skilled fishermen. Finally, much like mountains, the sea could also serve as a natural barrier and make them less-likely to face invasions.