The Theatre of Marcellus is an ancient open-air theatre, built in 13 BC and formally inaugurated in 12 BC by Augustus. At the theatre, locals and visitors alike were able to watch performances of drama and song. Today its ancient edifice in the rione of Sant'Angelo, Rome, once again provides one of the city's many popular spectacles or tourist sites.
The theatre was 111 m in diameter and was the largest and most important theatre in Ancient Rome; it could originally hold between 11,000 and 20,000 spectators. It was an impressive example of what was to become one of the most pervasive urban architectural forms of the Roman world. The theatre was built mainly of tuff, and concrete faced with stones in the pattern known as opus reticulatum, completely sheathed in white travertine. However, it is also the earliest dateable building in Rome to make use of fired Roman brick, then a new introduction from the Greek world.
The network of arches, corridors, tunnels and ramps that gave access to the interiors of such Roman theatres were normally ornamented with a screen of engaged columns in Greek orders: Doric at the base, Ionic in the middle. It is believed that Corinthian columns were used for the upper level but this is uncertain as the theatre was reconstructed in the Middle Ages, removing the top tier of seating and the columns.
The theatre fell out of use in the early 4th century and the structure served as a quarry for e.g. the Pons Cestius in 370 AD. However, the statues located inside the building were restored by Petronius Maximus in 421 and the remaining structure still housed small residential buildings.
In the Early Middle Ages the theatre was used as a fortress of the Fabii. In the 16th century, the residence of the Orsini, designed by Baldassare Peruzzi, was built atop the ruins of the ancient theatre.
Now the upper floors are divided into multiple apartments, and its surroundings are used as a venue for small summer concerts; the Portico d'Ottavia lies to the north west leading to the Roman Ghetto and the Tiber to the south west.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.