The Barrier Wall of Zul Qarnain
After having ascertained who Yajooj and Majooj were the next question to ascertain is the whereabouts of the Barrier built by Zul Qarnain before proceeding with the task of fixing the whereabouts of the barrier wall of Zul Qarnain one should bear in mind that the area of the devastation and plundering caused by Yajooj and Majooj was so wide that on the one side those who lived south of the Caocasion Mountains were greatly harassed while on the north and east side the people of China and Tibbet suffered the same fate. For this reason it so happened that various barrier walls were built at various times in history for safeguarding against their marauding hosts.
1. For example, their was the Great Wall of China which was almost a thousand miles long. It is also called the Mongalian Atkoda and in Turkish its name is "Bokorka."
2. A second barrier wall lies in the middle of Asia near the towns of Bukhara and Taraz at a place called Derbend. This wall was already present in the time of the Ameer Taimur Gorgani the great, and the special courtier of the Roman Emperor Silibar Girgirmany made mention of it in his book. And so also has it been mentioned in the account of his travels by Kalamchu the envoy of the King of Castile in Andalusia (Spain). This envoy visited Ameer Taimur Gorgani in 1403 A.D. on behalf of his King and he managed to pass the wall. He writes: "The wall of the iron door is situated on the Mosul route which leads from Samarkand to India."
3. A third barrier wall is situated in the Russain Territory of Daghistan. This is also called Derbend and is known as 'Babul Abwaab'. Some historians also call it Al-Baab (the door). Yaqoot Hamawy in his Mu'jamul Buldaan made a mention of it in detail and so did Idreesy in his Geography and so also did Bustani in his Da'iratul Ma'aarif. The sum total of their writings is:
"Derbend in Daghistan is a Russain city. It is situated on the Western shore of the Caspian Sea. Its latitude is 43.3° N and its longitude is 48.15° East. The town is also called Derbend Anusherwan, but it is best known as 'Babul Abwaab The town is surrounded by four such walls which ancient historians have called Abwaabul Baaniyah. Now these walls are all in a dilapidated state and is also called 'Babul Hadeed iron door because the walls have huge iron gates attached to them.'