The Mosque of Amr ibn al-As marks the site of Cairo's first mosque, founded in 642 after the Arab conquest of Egypt. The building has been rebuilt many times, so most of what stands today is later, but the site remains one of the key Islamic landmarks of Old Cairo.
Its large, open prayer hall is calmer than many central Cairo sights and gives useful context to the older layers of Fustat. It pairs well with the Coptic Quarter, the Babylon Fortress, and the Coptic Museum nearby.