The frontier.
The frontier established all things first and foremost. Cities developed around the development of frontier establishments. Cities depended on the raw goods that were most often harvested from the frontier or unsettled rural areas. As a specific example, the frontier provided what would become a major industry in the city of Chicago, the cattle industry. Animals were raised in the prairie states, shipped to Chicago for processing and distributed to the East, slaughterhouses made Chicago, built on a product of the frontier. The cities depended on the raw goods and resources to be built. And again, the only way major cities were settled, were first by being frontier encampments.