They get access to a unique building, representing the unique structure of Cahokia:

We also have plenty of events for Cahokia, that are largely rooted in the archaeological evidence we have of the civilizations, which notably collapsed just a few decades after the game starts, although it had been on a decline for decades. The last event shown can be accomplished after having been able to reverse this decline, and sets a new golden age for the people of Cahokia: