Greece
The physical geography of a place can cause some interesting things to happen.  The high, rugged mountains in Greece led to the isolation of its people.  What developed as a result, were many small city-states; meaning a city and all of the land it could control.  That means very little cultural diffusion happened.  In other words, very few ideas were making their way around the country. 

In turn, all that isolation led to the adoption of very different social, cultural and political laws and traditions.  Athens, for example, tried several forms of government before settling on a limited form of direct democracy.  Under this system, only eligible citizens were able to participate in government.

Athenian justice has sometimes been criticized as harsh and/or un-democratic because so many people were excluded from voting.  Women, slaves, men under a certain age, and all men who did not own land were barred from participation in Athenian government.  As you might expect, wealthy landowners controlled the politics, and probably very little changed.

Despite what appear to be shortcomings to the modern observer, Athens must be remembered as the first democracy in the western world.

Rome
For Rome, geography played a different role.  Most of southern Europe and north Africa were fairly easy to get to from Rome.  This allowed the Roman armies to conquer those places with relative ease.

The single greatest achievement of ancient Rome was its system of laws. Many of the basic ideas recorded in the Roman Laws of the Twelve Tables are still part of modern law practices—equality for all people under the law, the idea of being innocent until proven guilty, the right to defend yourself with evidence, and the right to know your accuser were all included.

Problems arose in the 200s CE when the huge Roman Empire was divided into two parts to make it easier to govern.  The western half declined in power, and the eastern half gained power.  The eastern portion continued to grow in importance, becoming known as the Byzantine Empire, after the capital city of Byzantium.

Byzantium
The Byzantine emperor Justinian (527-565 CE) is best remembered for organizing ancient Roman laws into a collection of civil laws known today as Justinian's Code.  It was so successful, that later rulers only added to it or updated it periodically. Justinian's Code became the foundation of medieval law, and was even used by the Roman Catholic Church. Today, international law still uses some concepts first assembled by Justinian.