Located on the eastern bank of the Tigris, and about 35 kilometers southeast of present day Bagdad, Ctesiphon was an ancient city of Iraq. In about 58 BC, during the Parthian Empire, it became the capital of Persia and remained the capital of the Sasnian Empire until the Muslim conquest of Persia in 651 AD. It is considered that, Ctesiphon measured 30 square kilometers, more than twice the surface of the fourth-century Imperial Rome.
The Taq-i Kasra, the enormous archway of Ctesiphon, is the only remaining structure of the ancient city. The arched rectangular vaulted  hall, open only on the facade side, was about 37 meters high, 26 meters across and 50 meters long, It is considered to be a landmark in the history of architecture, and is the largest single-span vault of brickwork in the world. It was a part of the Imperial Palace complex. The throne room behind it was said to be about 110 feet high, 80 feet wide and 160 feet long. It was a part of the Imperial Palace complex.
Ctesiphon was founded by Mithradates I, the king of the Parthian Empire, as a place of royal residence, after he annexed Babylonia by defeating the Greeks. During the late sixth and early seventh century, Ctesiphon flourished as one of the largest cities in the world. Merging with the surrounding cities along both the shores of River Tigris, including the Hellenistic city of Seleucia, it thrived into an opulent commercial metropolis. .
During the wars with the Roman Empire, Parthian Ctesiphon was defeated four times by the audacious Romans, and later once in the Sasanian period. It was also the site of the Battle of Ctesiphon, in which Julian the Apostate, the Roman Emperor from 361 to 363, was killed in action.  However, the Palace was improvised as a mosque by the Arabs, who captured the city of Ctesiphon in 637 AD. As luck would have it, with the flourish of the newly founded Bagdad, Ctesiphon lost its pride and was gradually abandoned. By the 8th century, it was completely replaced by Baghdad, and Ctesiphon’s deserted ruins were used as a source for building materials. As if that was not enough, subsequent floods destroyed the remaining structures of the great city, including Taq-i Kasra, one third of which was swept away by a flood in 1888.The most conspicuous structure remaining today is the great archway of Ctesiphon. The archway was once a part of the royal palace in Ctesiphon and is estimated to date between the 3rd and 6th centuries AD.  It is located in what is now the Iraqi town of Salman Park.
A German Oriental Society and University of Pennsylvania team excavated at Ctesiphon in 1928–29 and 1931–32, mainly on the western part of the site. In the late 1960s and early 1970s, an Italian team from the University of Turin also worked at the site, mainly doing restoration of the palace of Khosrau II.
Unfortunately, after the fall of Saddam Hussein its former museum was looted and vandalized ruthlessly without any consideration. The entire area was abandoned and battered by the rough weather. One of the symbolic structures of the early human civilization is now counting its days in a dilapidated condition. Though the Global Heritage Fund warned about the precarious state of the structure in 2004, only recently the Iraqi government has confirmed their plans for its restoration. However, the largest brick-built arch in the world, which is a reminder of one of the greatest cities of ancient Mesopotamia, is now surviving with the apprehension of being destroyed following decades of political unrest and threat from the Islamic State.
