The Iwan of Chosroes in Iraq is the only visible structure remaining of the Sassanid capital of Ctesphion (Madā’in in Arabic), about 35 km south of present-day Baghdad. Its Iwan, or arch, the largest vault of unreinforced brickwork in the world, is considered an architectural marvel. Possibly constructed during the reign of Anushirwan (Chosroes I)…