Several historical figures, both Muslim and Christian, searched for Alexander's Gate and several different identifications were made with actual walls. During the Middle Ages, the Gates of Alexander story was included in travel literature such as the "Travels of Marco Polo" (1254–1324 AD) and the "Travels of Sir John Mandeville". The "Alexander romance" identified the Gates of Alexander, variously, with the Pass of Dariel, the Pass of Derbent, the Great Wall of Gorgan and even the Great Wall of China. In the legend's original form, Alexander's Gates are located at the Pass of Dariel. In later versions of the Christian legends, dated to around the time of Emperor Heraclius (575–641 AD), the Gates are instead located in Derbent, a city situated on a narrow strip of land between the Caspian Sea and the Caucasus mountains, where an ancient Sassanid fortification was mistakenly identified with the wall built by Alexander. In the "Travels of Marco Polo", the wall in Derbent is identified with the Gates of Alexander. The Gates of Alexander are most commonly identified with the Caspian Gates of Derbent whose thirty north-looking towers used to stretch for forty kilometres between the Caspian Sea and the Caucasus Mountains, effectively blocking the passage across the Caucasus. Later historians would regard these legends as false: