Annalee Newitz has a detailed and fascinating story in Ars Technica about the Cahokia site, on the Illinois side of the Mississippi River from St. Louis: “Finding North America’s lost medieval city”.
At the city's apex in 1100, the population exploded to as many as 30 thousand people. It was the largest pre-Columbian city in North America, bigger than London or Paris at the time. Its colorful wooden homes and monuments rose along the eastern side of the Mississippi, eventually spreading across the river to St. Louis. One particularly magnificent structure, known today as Monk’s Mound, marked the center of downtown. It towered 30 meters over an enormous central plaza and had three dramatic ascending levels, each covered in ceremonial buildings. Standing on the highest level, a person speaking loudly could be heard all the way across the Grand Plaza below. Flanking Monk’s Mound to the west was a circle of tall wooden poles, dubbed Woodhenge, that marked the solstices.
Despite its greatness, the city’s name has been lost to time. Its culture is known simply as Mississippian. When Europeans explored Illinois in the 17th century, the city had been abandoned for hundreds of years. At that time, the region was inhabited by the Cahokia, a tribe from the Illinois Confederation. Europeans decided to name the ancient city after them, despite the fact that the Cahokia themselves claimed no connection to it.
She reports from the scene where she participated in some archaeological work, including some of my favorite bits: digging in the houses and structures used by ordinary people, on the outskirts of the monumental area.