Public domain, courtesy of Jorge Barrios
At a certain time during the Dark Ages, there were no cities in most of Europe, but just thousands of farms, spread across the clearings and meadows of Europe, and old Roman roads built long ago, crisscrossing between them. At the junctions of these roads, there might be ruins, that would go unnoticed as would whatever tiny, insignificant traffic crossed through them. The ports didn't operate in Europe, even as far away as Italy and Turkey, because of Viking raids, except in a few areas like Ireland, Danegild, and Normandy where the Norse set up there own, and the regions became wealthy as a consequence. In those days, all was done out of the manor house, and trade was very limited. Manors were extremely, bitterly poor, and highly self-sufficient, and noble lords had extreme authority and martial responsibility.
But then technology, bit by bit, began to improve. Castles allowed for the defense of harbors, opening the ports and the seas. Modern quarying developed, and with it a supply of marble. Trade with the Islamic empire brought spices and coffee to the new ports, while increases in population and the size and number of manor houses brought about an ecological disaster, the destruction of Europes forests, and with it the need to travel ever further for lumber and its replacements, clay brick and coal.
At the intersections of roads, you have the fastest route on average to anywhere. While going down the road your travel time to one particular place would decrease, for a few limited industrial enterprises that is indeed good enough, your travel time to all the other places those roads go to will increase so much more. Further, more customers and workers and material will cross through the intersection then anywhere else. Lodges, markets and taverns would set themselves up first, and then manufacturers and stables and warehouses would set up near the market, and with all of these little brick houses would sprout along every alleyway and available road as workers too chose the fastest transportation for their life.
Eventually, highly specialized businesses, utilizing the economic base of the city, would set up as well. These would include law offices, accountants, computers (in that day meaning people who would crunch numbers for you), moneylenders, and craftspeople like blacksmiths serving both sword and plowshare. Church and Government too would notice the city, and the workers and citizens serveable through it, setting up bureaucracy and courthouses, which in the dark ages didn't exist, along with fortresses, great cathedrals, universities and libraries, and comissioning artists and clockmakers to celebrate their authority in great squares and plazas. Criminals too would benefit from the economic base of the city, utilizing its many highways, services and people, as would entertainers.
The reality is that cities are a giant device for the conservation of time, by quickening transportation from one place to another, and the conservation of energy, by decreasing the distance that freight and people must travel to get to its customers and clients. Cities, in the end, are the product of their transportation, and the customers, employers, industry and resources they afford within a reasonable distance. If a city loses this, it has nothing, and is only an overpriced piece of land, but it is because of the ability to save people time, and as a result, allow them to live more, and do more, that people will pay 300 times as much as the same quanity of land in the country to own it. In the words of Ali from Burlesque, when talking about why she left Iowa, "I looked around and realized that I didn't see anyone I wanted to be." You can be more and do more in a city because where you go to do what you want to do is reachable in reasonable time.
Now in the recent past, people would live sixty miles out of the city and commute to wherever it was they worked. When people lived and worked in one place their entire lives, that made sense, but in a world where people change jobs on average every 2 years, it makes sense to be as close to as many employers as possible, and since industrial employers will opt for cheaper land along whatever railroad line leads to their suppliers, while extremely common businesses (e.g. drugstores) will opt for more minor, inexpensive junctions that they have the capacity to serve, not all of them will be in the big city. As we go in to the future, we find the employees act more like businesspeople, and the need for speed (and with it, the city) becomes greater then ever.
Some cities, like Seattle, are still stuck in the past imagining that transportation is just a quality of life issue, something we can tinker around with in social engineering and throw immesaurable costs on to, but across the lake from Seattle, Bellevue, on I-405, the eastern bypass of I-5, and the third city eastward on I-90, has expanded and improved its interstates, with its own money. The result has been a building boom, dozens of new towers, 40 stories tall, going up and new businesses moving in. Meanwhile, Seattle has lost its largest bank, Washington Mutual, to the financial slowdown, and no one seems quite set to replace it. But they still continue to toy with forcing people on to trains that zig zag at 35 mph instead of heading straight where they go. Voters constantly complain about transportation expense, but what they don't realize is that transportation is the heart and soul of their city life, is what allows them the opportunities they have.