Arch of Ctesiphon
Lonely Planet review
Little is left of the city of Ctesiphon, apart from the arch. The city was built in the 2nd century BC by the Parthian Persians. The arch was part of a great banqueting hall and, apparently, is the widest single-span vault in the world. It survived the disastrous flooding of the Tigris in 1887, which destroyed much of the rest of the building.