Hamlets are predominantly agricultural areas with some sparse human settlements, while villages are small aggregates of dwellings forming communities. Towns are larger aggregates of dwellings, while cities are even larger settlements. A megalopolis is formed when when urban expansion results in an overlap in development by cities in close proximity to one another, resulting in a network of high-density human settlements.

A surplus of food was necessary for the establishment of the world’s earliest cities. A surplus of food means that there is enough food to support population growth and, just as importantly, to support non-agricultural laborers and citizens. In order for buildings to be constructed, government to emerge, and a standing army to arise in a city a society must be able to support a proportion of the population in non-agricultural work.

The Feudal System, which prevailed in Europe and other parts of the world throughout the Medieval Period, significantly hindered the growth of cities. In order for Feudalism to work, a large proportion of the population has to be tied to the land in the form of serfs, or peasant farmers. This hinders urbanization because there is little surplus labor and no established middle class.

Mexico City, the largest city in Mexico and one of the most densely populated cities in the world, rests on the same site as the capital city of the Aztec Empire, called Tenochtitlan.

During the Renaissance Era of European history, European cities experienced dramatic growth. Urbanization took place at a fast pace and colonial riches began to support a burgeoning middle class. However, throughout the rest of the world the impact of European conquest and disease led to sporadic and sometimes disastrous decline for many cities and civilizations. Cities like Tenochtitlan in modern-day Mexico and Cusco in modern-day Peru are two classic examples.

Human civilization began with the Neolithic Revolution, also called the First Agricultural Revolution, several thousand years ago. The first cities arose in close proximity to agricultural hearths like the Nile River because these regions could provide enough food to sustain a large population.

Agriculture occurred mainly in rural areas surrounding the ancient city. Agricultural producers could then bring their crops and animal products to the ancient city to sell or trade, but they would not produce within the ancient city.