Qarnayn, in the mountains of Caucasia, in the region of Darband, to halt the inroads of the plundering and pillaging Tatar peoples. There are very many barriers of this sort. Since the All-Wise Qur’an speaks with all mankind, it mentions what is apparently a particular incident, and recalls all events similar to it. It is from this point of view that the narrations differ concerning the Barrier and Gog and Magog, as well as the writings of the Qur’anic commentators about them.
Furthermore, the All-Wise Qur’an switches from one event to another distant one due to the association of ideas. The person who fails to think of this association supposes the two events to be close in time. Thus, the Qur’an’s predicting the end of the world from the destruction of the Barrier is not because the two events are close in time, but to make two subtle points connected with the association of ideas. That is, the world will be destroyed just as the Barrier will be destroyed. Also, just as mountains, which are natural divine barriers, are firm and will be destroyed only at the end of the world; so the Barrier is firm as a mountain and will be levelled to dust only at the destruction of the world. Even if it suffers damage from the assaults of time, it will mostly remain intact. Yes, the Great Wall of China is one particular meaning of the universal meaning of the Barrier of Dhu’l-Qarnayn and has been standing for thousands of years and is still there for all to see. It is read as a long, petrified, meaningful line from ancient history, written by man’s hand on the page of the earth.
YOUR THIRD QUESTION
Brief replies concerning Jesus (Upon whom be peace) killing the Dajjal are found in both the First and the Fifteenth Letters, which should suffice you.