I visited Cahokia Mounds a few years ago and was thoroughly impressed not just with the size of these massive earthworks but also with the knowledge of the staff. This is unusual as I have usually found some error in the information presented at other such parks.
This ancient metropolitan area flourished between 700 and 1400 AD. Some earthworks were used for burials other flat top pyramids were used for homes for the elite. The centerpiece of the complex is Monk’s Mound, a 100-ft flat top pyramid, the tallest in the US.
Another interesting feature is Woodhenge, a circle of posts used for astronomy.
I stayed at Horseshoe Lake State Park. I was unable to get into the campground so I parked in a parking lot. Surprisingly no one asked me to leave. This is a beautiful, quiet park with excellent views of the lake and access for water sports. There is also a great view of the Saint Louis skyline.