The remains of the roman Arena can be visited
Teatro di Marcello

In Rome, the Teatro di Marcello is an ancient structure that serves as a residential building to this day. Originally a theater that held thousands of spectators, it now presents itself as a blend of history and modern life.
History+
Julius Caesar had already acquired the land north of the vegetable market, near the Temple of Apollo Sosianus. Augustus had the theater built there, completed in 13 BC and dedicated to his nephew Marcellus, who had died prematurely. Two smaller temples had to be demolished for its construction. Unlike Greek theaters, which were often integrated into natural slopes, the Theater of Marcellus was a freestanding structure. With a height of approximately 33 meters and a capacity of 10,000 to 15,000 spectators, it was Rome's largest theater and served as a model for the later Colosseum.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: http://commons.wikimedia.org/wiki/File:Teatro_Marcello_Roma_HDR_2013_03.jpg
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 41.8920, 12.4795