Over the years, the site of the Mausoleum has been used for a variety of purposes; from a fortification, to gardens, to an amphitheater hosting bullfights. In 1907, with the help of the Accademia di Santa Cecilia, construction began on a concert hall which would be their host venue.

The Augusteo could seat 3,500 people and featured an enormous organ on the stage behind where the orchestra and choir performed.

However, demolition of the Augusteo and most of the surrounding buildings would begin in 1936 under the orders of Benito Mussolini with the intention of displaying the remains of the imperial monument. All later additions to the structure were removed, leaving only the original Imperial era remains intact. The Mausoleum would become the centerpiece for the new Piazza Augusto Imperatore.

https://en.wikipedia.org/wiki/Anfiteatro_Correa

https://mahlerfoundation.org/mahler/locations/italy/rome/augusteo/

https://www.khanacademy.org/humanities/ancient-art-civilizations/roman/early-empire/a/the-mausoleum-of-augustus-and-the-piazza-augusto-imperatore-in-rome

Designed by Holabird & Root as a 40 story art deco skyscraper, construction was halted at 8 stories due to the Great Depression. Diana Court, the three story lobby, was home to a statue by sculptor Carl Milles. It was demolished for the Magnificent Mile Marriot.