What is the importance of Circus Maximus in the nonfiction book, The Early History of Rome: Books I-V?

The Circus Maximus was the first public outdoor stadium built in the city of Rome. This was created during the reign of the kings. It comes up in the first and second volume of the book. The author points out to readers that when he wrote the five volumes about early Rome, the Circus Maximus was still the biggest stadium known.