"The city's inhabitants built over 100 mounds, eighty of which remain. One of them still towers 92 feet over the surrounding fields and is easily visible from the scratched postage-stamp windows of St. Louis' Gateway Arch. With somewhere between 10,000 to 15,000 people, it held the record for the largest American city until around 1800, when Philadelphia finally overtook it.
With that many people crammed into just under three-quarters of a square mile-the estimated size of the city's neighborhoods-it may sound like Cahokia was as cramped as the slums of Upton Sinclair's Chicago. But it probably didn't feel that way. Sweeping plazas and towering mounds added nearly three square miles of open space, keeping much of the city open and airy like Baron Haussmann's Paris. Yet unlike the city on the Seine's astronomical modern density of 58,890 people per square mile, Cahokia's population lived at a positively suburban 1,000 to 1,500 people per square mile, thanks to the plazas and mounds."
Based on archaeological excavations, just 20% of the city's land was dedicated to housing.