 Opar reappeared in further Tarzan novels and was later taken up in the Khokarsa novels of Philip José Farmer and various derivative works in other media.Burroughs also introduced other lost cities in various hidden corners of Africa, for Tarzan to visit, such as a valley inhabited by stray Crusaders still maintaining a Medieval way of life.Robert E. Howard's Conan the Barbarian also visited several lost cities, and Lee Falk's "The Phantom" was initially written in this genre.