Christian legends speak of the Caspian Gates (Gates of Alexander), also known as Alexander's wall, built by Alexander the Great in the Caucasus mountains. Several variations of the legend can be found. In the story, Alexander the Great built a gate of iron between two mountains, at the end of the Earth, to prevent the armies of Gog and Magog from ravaging the plains. The Christian legend was written in Syria shortly before the Quran's writing and closely parallels the story of Dhul-Qarnayn. The legend describes an apocryphal letter from Alexander to his mother, wherein he writes: