Ports or Trade Centers.
The 5 largest cities in the US in 1800 were New York, Baltimore, Charleston, Boston, and Philadelphia.
Each of these cities grew because they had good trade connections. New York, Boston, and Baltimore all specialized in manufactured goods going to and from Europe and to the South. Charleston was connected to the slave trade as well as exports from the plantations. Philadelphia was the trade connection between the east coast and the western frontier.
