City (n.) 1. A large urban center that is built by successive waves of upwardly-mobile immigrants and run by generations of political elites that hate immigrants. 2. A large collection of buildings maintained at expense to be optimal homes for cockroaches and rats.
In 2008, over half the world's people live in cities. While this creates somewhat of a mess in otherwise nice parts of the world, one should remember that urban life is ecologically more efficient than rural life, at least for the same level of consumption. In theory at least, city life offers a more sustainable future than rural life.
The oldest city states based only on agriculture had an obvious weak point – the climate and its tendency to change like the weather – and prehistory is littered with the remains of once-great city states that grew and popped like bubbles. City states based on trade are much more flexible and robust, and the great cities and empires of the last two to three thousand years have been based primarily on control of trade routes rather than agriculture.
The agricultural village is only stable in certain conditions. The key factor is probably the ease with which young, disaffected men and women can leave the place to start-up their own. When this is easy, villages lose their people to towns and cities. When this is hard, people stay put and villages prosper. Waterways and open country will therefore encourage the growth of larger settlements at the expense of villages. Rivers or routes across desert and sea, carrying people and goods… these are the blood supply of towns and cities.
Cities grow spontaneously, following Zipf's Law, but there is always someone in charge. Successful cities levy taxes on all trade and commerce, and invest this in expansion, infrastructure, defense, and wars. So, it's natural that the rulers will take control of the money supply, to protect it and to manipulate it. One cannot easily tax peasants on their annual chicken production. But it is easy to tax merchants involved in trade. Money-based trading societies are brutally efficient, flexible, and scalable, and have thus come to dominate the human world.
While a village society has a status hierarchy from strongest, prettiest, and smartest down to weakest, ugliest, and stupidest, a city society has a class system, reinforced by the permanent influx of poor immigrants from the villages. Often this is split along extended family lines: origin, language, skin tone. Most mature city societies have a clear class system, with the oldest families controlling most of the property and wealth, the middle classes with their connections abroad controlling most of the business and trade, and the recently arrived poor acting as the exploited workforce. This is the case in our modern cities as much as it was in the great cities of the past.
Power in city-based societies lies with those who control its financial systems. Money has always been seen as the expression of power. Indeed, money can only exist by the grace of an issuing and controlling power. When the Roman empire wanted to pay its soldiers in salt, it first took control of all salt production. When West African empires used the cowrie shell as money, their rulers controlled access to the cowrie beaches. When a modern-day general wants to seize power, one of the first things he does is take control of the central bank.