Greece was a really mountainous peninsula. The mountains isolated many city-states making many different cultures. This meant that there was really any trade of ideas or products between the inhabitants of Greece. If they needed to unify, it would be tough. The Peninsula made it so they could trade with other civilizations overseas.
Rome was built on 7 hills and was in the middle of the Mediterranean. Rome was a peninsula and was naturally protected by water and mountains. To the North there was the Alps mountains and the other sides was water. This gave Rome a connection to many other lands to trade with and conquer.
