(18:98) Zul-Qarnain said, "This is a mercy from my Lord; but when the time of my Lord's promise shall come, He will level it to dust *71 and the promise of my Lord is true. " *72

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*71) That is, "Though I have built a very strong iron-wall, as far as it was possible for me, it is not ever-lasting, for it will last only as long as Allah wills, and will fall down to pieces when the time of my Lord's promise shall come. Then no power in the world shall be able to keep it safe and secure."
As regards the time of Allah's promise, it has two meanings: (1) It may mean the time of the destruction of the wall, and (2) it may also mean the time of the death and destruction of everything destined by Allah at the end of the world i.e., the Hour of Resurrection.
Some people have entertained the misunderstanding that the wall attributed here to Zul-Qarnain refers to the famous Wall of China, whereas this wall was built between Derbent and Dar'yal, two cities of Daghestan in the Caucasus, the land that lies between the Black Sea and the Caspian. There are high mountains between the Black Sea and Dar'yal having deep gorges which cannot allow large armies to pass through them. Between Derbent and Dar'yal, however, there are no such mountains and the passes also are wide and passable. In ancient times savage hordes from the north invaded and ravaged southern lands through these passes and the Persian rulers who were scared of them had to build a strong wall, 50 miles long, 29 feet high and 10 feet wide, for fortification purposes, ruins of which can still be seen. Though it has not yet been established historically who built this wall in the beginning, the Muslim historians and geographers assign it to ZulQarnain because its remains correspond with the description of it given in the Qur'an. Ibn Jarir Tabari and Ibn Kathir have recorded the event, and Yaqut has mentioned it in his Mu jam-ul-Buldan that when after the conquest of Azerbaijan, Hadrat `Umar sent Suraqah bin `Amr, in 22 A.H. on an expedition to Derbent, the latter appointed `Abdur Rehman bin Rabi`ah as the chief of his vanguard. When 'Abdur Rehman entered Armenia, the ruler Shehrbraz surrendered without fighting. Then when `Abdur Rehman wanted to advance towards Derbent, Shehrbraz informed him that he had already gathered full information about the wall built by Zul-Qarnain, through a man, who could supply all the necessary details and then the man was actually presented before `Abdur Rehman. (Tabari, Vol. III, pp. 235-239; AI-Bidayah wan-Nihayah, Vol. VII, pp. 122-125, and Mu jam-ul-Buldan, under Bab-ul-Abwab: Derbent).
Two hundred years later, the Abbasid Caliph Wathiq (227-233 A.H.) despatched a party of 50 men under Sallam-ul-Tarjuman to study the wall of ZulQarnain, whose observations have been recorded in great detail by Yaqut in Mu jam-ul-Buldan and by Ibn Kathir in AI-Bidayah. They write that this expedition reached Samarrah from where they reached Tiflis (the present Tbilisi) and then through As-Sarir and Al-Lan, they reached Filanshah, from where they entered the Caspian territory. From there they arrived at Derbent and saw the wall. (AIBidayah Vol. II, p. 111, Vol. VII, pp. 122-125; Mu jam-ul-Buldan: under Bab-ulAbwab). This clearly shows that even up till the third century of Hijrah the Muslim scholars regarded this wall of the Caucasus as the wall of Zul-Qarnain.
Yaqut in his Mu jam-ul-Buldan has further confirmed the same view at a number of places. For instance, under Khazar (Caspian) he writes:
"This territory belongs to the Turks, which adjoins the Wall of ZulQarnain just behind Bab-ul-Abwab, which is also called Derbent." In the same connection, he records a report by Ahmad bin Fadlan, the ambassador of Caliph Al-Muqtadar-billah, who has given a full description of the Caspian land, saying that Caspian is the name of a country whose capital is Itil (near the present Astrakhan) right through which flows River Itil, which joins the Caspian front Russia and Bulghar.
Regarding Bab-ul-Abwab he says that this city is called both Al-Bab and Derbent, which is a highly difficult passage for the people coming from the northern lands towards the south. Once this territory was a part of the kingdom of Nausherwan, and the Persian rulers paid particular attention to strengthening their frontiers on that side.
*72) Here the story of Zul-Qarnain comes to an end. Though this story has been related in answer to the questions put by the disbelievers of Makkah as a test along with the stories of the "Sleepers of the Cave" and "Moses and Khidr", the Qur'an has utilized this story, too, for its own aim and object, as if to say, "ZulQarnain, about whose glory you have heard from the people of the Book, was not merely a conqueror, but also a believer of the doctrines of Tauhid and the lifeafter-death and acted upon the principles of justice and generosity. He was not a mean person like you who have been puffed up by the possession of petty estates, and give yourselves airs of superiority."