The most prominent structure is Monks Mound, rising ten stories high at the center of the complex and fronting on a Grand Plaza. Monks Mound is the largest Pre-Columbian earthwork in the Americas, and the complex is the largest earthwork north of Mexico. The engineering of the mounds showed that their builders had an expert knowledge of the varying soils and their capacities. Cahokia was a complex, planned, and designed urban center with a residential population, farming, and artisan production of refined crafts and goods. With its location at the confluence of three major rivers, it was the center of a regional trading network reaching to the Great Lakes and the Gulf Coast. With a population estimated at 30,000 at its peak, Cahokia was the largest city north of modern-day Mexico. Perhaps for ecological reasons—deforestation and overhunting—the city went into decline after 1300 and was abandoned before 1400. No city in the territorial United States surpassed this population until after 1800, when Philadelphia exceeded it.