Their trading network extended from the Great Lakes to the Gulf Coast, and into the Southeast of the present-day United States.

The chief center of a Middle Mississippian settlement is at Cahokia, in present-day Illinois, a city that at its peak had 20,000–30,000 people.

These settlements are characterized by the construction of earthwork mounds, stockades, and houses, by decorated Mississippian culture pottery, and agricultural practices.

For example, items found at the settlement include copper from Michigan's Upper Peninsula, seashells from the coast of the Gulf of Mexico, and stone such as Mill Creek chert from other areas of the Midwest.

The dwellings were built around a central ceremonial plaza likely used for rituals and public gatherings, as has been found at similar locations.

Inside, a single family slept on pole-frame beds, covered with tamarack boughs, deer skins, and furs.

Storage pits dug in the earthen floor of the house held foods such as corn, nuts, and seeds in woven bags.

The staple of the diet was cultivated corn (maize), and other plants were also gathered as food, such as acorns, hickory nuts, and berries.

They hunted birds and turtles, collected mussels, and caught fish in the Crawfish River directly next to the site.

[6] Trees nearby provided wood for posts for house walls and stockades, bows and arrow shafts, and firewood.

[6] The most obvious features of Aztalan are its pyramid-shaped, flat-topped platform mounds and the stockades, believed to have served both ceremonial and defensive functions.

[7] A special structure, approximately 4 by 2 metres (13.1 by 6.6 ft), with its long axis towards the northeast–southwest, was built on the west side of the mound.

The eastern mound had a large open-walled structure, about 12 by 27 metres (39 by 89 ft), built on top of it, with firepits lined with white sand inside.

The settlement was surrounded on the north, west, and south sides by a palisade, a wall of logs set vertically into the ground.

The stockade was finished by people weaving flexible willow branches through the posts, and plastering the whole with a mixture of clay and grass to fill in the gaps, a technique similar to wattle and daub.

[8] It had at least 33 square bastions at regular intervals along its length, similar in form and placement to some European fortifications, to allow defensive warriors to cover that area by shooting arrows.

A young man named Timothy Johnson discovered the ruins of the ancient settlement in December 1835.

However, in 1838, President Martin Van Buren refused a request by Massachusetts statesman Edward Everett to withdraw the site from public sale, and it was sold for $22.

In the following years, the surface was plowed, the mounds were leveled for easier farming, pottery shards and "Aztalan brick" were hauled away by the wagonload to fill in potholes in township roads, and souvenir hunters took numerous artifacts.

They made their first purchase of some of the land in 1921, three acres (12,000 m2)) west of the stockade and containing eight conical mounds, and presented it to the Wisconsin Archeological Society.

In 1941, the newly founded Lake Mills-Aztalan Historical Society began an energetic campaign to preserve the stockade area.