How did the geography of Ancient Greece shape their civilization?

With arable land split up into river valleys, separate groups formed the basis of what became city-states - a tract of farming land with a citadel for protection which developed into a city. Having developed as separate entities with their own government, these city-states had no incentive to combine into larger states, and preserved their independence rigorously.
Ancient Greece was a rocky place with little land to farm on, so they took to the sea and shipping. They conquered new lands, found new foods, learned new things and became the traders of the Mediterranean. All of this shaped their culture and society as well as spreading their culture, religion, thinking, and philosophy to the ancient world.