Cairo Citadel, Egypt. Also known as the Saladin Citadel after the Ayyubid ruler Salah-al-Din, who ordered its construction. Completed in 1183-4 CE it remained the seat of government for Egypt until the 1860s. Now part of the Historic Cairo World Heritage Site. Photographed from 3000 feet in December 1918, this image occupies half of a glass plate negative.