Tāq Kasrā (), also transcribed as Taq-i Kisra or Taq-e Kesra (, romanized: tâğe kasrâ) or Ayvān-e Kesrā (, meaning Iwan of Chosroes) are the remains of a Sasanian-era Persian monument, dated to c. the 3rd to 6th-century, which is sometimes called the Arch of Ctesiphon. It is located near the modern town of Salman Pak, Iraq. It was the facade of the main palace in Ctesiphon, and is the only visible remaining structure of the ancient capital city. The archway is considered a landmark in the history of architecture, and is the second largest single-span vault of unreinforced brickwork in the world after Gavmishan Bridge. History The exact time of construction is not known with certainty. Some historians believe the founder is Shapur I who ruled Iran from 242 to 272 AD and some other believe that construction possibly began during the reign of Anushiruwan the Just (Khosrow I) after a campaign against the Byzantines in 540 AD. The arched iwan hall, open on the facade side, was about 37 meters high 26 meters across and 50 meters long, the largest man-made, free standing vault constructed until modern times. The arch was part of the imperial palace complex. The throne room—presumably under or behind the arch—was more than 30 m (110 ft) high and covered an area 24 m (80 ft) wide by 48 m (160 ft) long. The top of the arch is about 1 meter thick while the walls at the base are up to 7 meters thick. The catenary arch was built without centring. In order to make this possible a number of techniques were used. The bricks were laid about 18 degrees from the vertical which allowed them to be partially supported by the rear wall during construction. The quick drying cement used as mortar allowed the fresh bricks to be quickly supported by those that were previously laid. The Taq Kasra is now all that remains above ground of a city that was, for nine centuries—from the 2nd century BC to the 7th century AD—the main capital of the successor dynasties of the Persian empire: Parthians and Sassanids. The structure left today was the main portico of the audience hall of the Sassanids who maintained the same site chosen by the Parthians and for the same reason, namely proximity to the Roman Empire, whose expansionist aims could be better contained at the point of