4. When one proceeds from this Babul Abwaab towards the west towards the Interior of Caucasia one comes across a pass which is known as Daiyal Pass which is situated in the highest parts of Caucesia. There we find another smaller wall much is called Qafqauz or Mount Kauka or Mount Qauf pass. This wall has been made between two mountains. Bustani writes about it thus:
"And near it is another barriers wall on route towards the western side, which the Persians probably built for protection against the Berbery from the north. The builder of this wall could not be ascetained. Some attributed it to Alexander the Great and some attributed it to Kisra (Caeser) or to Nausherwan. Yaqoot says this wall was prepared with melted copper and iron. In Encyclopaedia Britanica under the heading Derbend this iron wall has been described in more or less similar terms."
Because all these wall have been built towards the north side and because they were all built for the same purpose, there is quite a bit of doubt as to which is the wall of Zul Qarnain. Therefore we also find among the historians much difference of opinion is establishing. This difference of opinion has lead to an interesting situation: The name Derbend is connected to two of these places and both places have a barrier wall and the reason for the building of the wall was the same.
Hence leaving aside the great wall of China, the question arises as to which of the three walls is the wall built by Khorus (Zul Qarnain) and which Derbend is the right one.
The Qur'an and the Wall of Zul Qarnain
With regard to the wall built by Zul Qarnain the Qur'an mentions two important facts. The first is that it was built between two mountains and though it the pass between the two was closed up, through where the tribes of Yajooj and Majooj used to come to weak have and plunder upon other tribes of the area. As the Qur'an states: