Theatre of Marcellus

Teatro di Marcello
The building was erected on the southern border of the IXth Augustan territory, Campo Marzio, on the site traditionally dedicated to representations. From 179 BC, there was the theater and proscenium in Apollinis, connected to the Temple of Apollo.
The theater was begun by Julius Caesar, who expropriated large areas of the site and demolished the existing buildings, including the Temple of the Pietà, for which he was widely criticized.
Augustus resumed the project and carried out further expropriations to enlarge the area and erect a larger building. Probably completed as early as 17 BC, it was used for secular games and consecrated in 13 or 11 BC, giving it the name of Marcellus, the emperor's designated nephew, who died in Baia in 27 BC.
In the 1500s, Baldassarre Peruzzi, commissioned by the Savelli family, built the palace that still stands today. It was purchased by the Orsini family two centuries later. The lower part, which corresponds to the Roman structures, was acquired, excavated, and restored by the city of Rome in the 1530s after the area had been radically liberated. The Theater of Marcellus was a grandiose structure with a diameter of 130 meters, in which the Roman-type theater was built in its full form: a complete building with a solid architectural unity, not bound by any topographical or orographic requirement like the Greek theater.
In the axial space of the lower cave, the vault was covered with white stucco in rounds and octagons decorated with figures.
The key arches of the two lower floors were decorated with colossal marble masks referring to tragedy, comedy, and satirical drama.
The cavea, which has not been excavated, was divided radially into six zones and horizontally into three, beyond the proedria at ground level. The capacity was calculated at 15,000 seats, or 20,000 if necessary.
From fragments related to the theater, we know of other elements of the marble floor plan of ancient Rome; the semicircular orchestra was accessed from a rectilinear parodoi.
Text/Source: Comunicazione Sovrintendenza, Piazza Lovatelli 35, Rome
sovraintendenzaroma.it/i_luoghi/roma_antica/monumenti/teatro_di_marcello

You can still clearly see parts of the outer wall of the Teatro di Marcello,
completed in 13 B.C. It is the only remaining theater besides the Colosseum. Named
after the early death of Marcellus (20 years old), son of Octavia, sister of Emperor Augustus. More than 20,000 spectators could enter. It was also Rome's first theater. The
similarity with the later built Colosseum is striking. After all, it served as a model. The theater was used in the Middle Ages as the home of the Fabi and later Savelli families.
During the 16th century, a palace was built, the Palazzo Orsini, on the site where the theater seats once stood.

Because of its appearance, the Marcellus Theater is often confused with the Colosseum. It's much older though. It was already begun by Caesar and continued by Augustus 11 BC. BC, who dedicated it to his late nephew Marcellus.