Built Without a Single Nail: How the World's Largest Brick Vault Has Stood for 1,400 Years, Surviving Empires and Wars (14 Photos)

The ancient city of Ctesiphon, sprawling along the banks of the Tigris River roughly 35 kilometers southeast of modern-day Baghdad, was founded in the late 120s BCE.





It grew into one of the grandest cities of late antiquity in Mesopotamia and stood as the world's largest metropolis from 570 CE until its fall during the Muslim conquest in 637 CE.



The sole surviving structure of Ctesiphon today is the majestic vaulted hall of Taq Kasra, which served as the palace of Sasanian King Khosrow I in the late 6th century. The arch remains one of the world's largest single-span unreinforced brick vaults.





Ctesiphon was founded by Mithridates I, King of the Parthian Empire, as a royal residence after he annexed Babylonia following his victory over the Greeks. Under Parthian rule, Ctesiphon developed into a major political and commercial hub, officially becoming the imperial capital by 58 BCE. Over time, the city merged with Seleucia—the former Hellenistic capital—and surrounding settlements to form a thriving cosmopolitan metropolis.



Mithridates I

In the 2nd century, a power struggle between the Romans and the Parthians saw Ctesiphon change hands six times. During their final occupation, the Romans sacked and devastated the city, destroying its palaces and enslaving thousands of its inhabitants.



In 226 CE, the city fell to the Sasanian Empire, ushering in a new era of prosperity. The oldest inhabited districts of Ctesiphon lay on its eastern side, referred to in Arabic sources as the Old City, home to the main Sasanian residence known as the White Palace.



The southern quarter of Ctesiphon was known as Asbanbar, famous for its grand halls, lavish riches, public games, royal stables, and bathhouses.



Khosrow I

Following another brief Roman occupation, the Sasanian king Khosrow I reclaimed Ctesiphon and commissioned a grand palace complex featuring a massive vaulted arch standing 37 meters high and spanning 26 meters across.



The arch measures roughly 1 meter thick at its crown, while the supporting walls at the base reach a thickness of up to 7 meters. Until the modern era, it stood as the largest unbraced vault in the world.



After capturing Ctesiphon in 637 CE, Arab forces converted the palace into a mosque until the area was gradually abandoned. By the 8th century, Ctesiphon was eclipsed by the newly founded city of Baghdad, and its deserted ruins were scavenged for building materials.



Subsequent flooding destroyed most of the remaining structures, including Taq Kasra, a third of which was swept away by a devastating flood in 1888.



The imposing brick ruins of Taq Kasra are all that remain above ground of a city that served as the primary capital for Iranian dynasties for over seven centuries.



Taq Kasra sits on the outskirts of the modern Iraqi town of Al-Mada'in (in the Salman Pak area), about 30 to 35 kilometers southeast of Baghdad. While the arch remains the only visible above-ground remnant of ancient Ctesiphon, its condition is cause for grave concern. The northern wing, restored in the 1970s, has suffered extensive weather damage.



Following heavy rainfall in 2020, bricks from the modern restoration work began to collapse. Experts noted that modern cement patches had destabilized the original ancient structure. Starting in 2026, Iraqi experts alongside Italian colleagues have been preparing a major conservation project, while Iran has also expressed readiness to assist in its restoration. Though the site remains open to visitors, it urgently requires critical stabilization work.