His Journey to the Land of Ya'juj and Ma'juj, and building the Barrier
Allah says of Dhul-Qarnayn:
ثُمَّ أَتْبَعَ سَبَباً
(Then he followed (another) way) meaning, he traveled from the east of the earth until he reached a place between the two mountains which were next to one another with a valley in between, from which Ya'juj and Ma'juj (God and Magog) will emerge into the land of the Turks and spread mischief there, destroying crops and people. Ya'juj and Ma'juj are among the progeny of Adam, peace be upon him, as was recorded in the Two Sahihs;
1. The word السَّدَّ used in the expression: بَيْنَ السَّدَّيْنِ denotes what be-comes a barrier against something. It may be a wall or mountain and it may be natural or artificial. At this place, the word: السَّدَّيْنِ (as-saddain) means two mountains that stood as a barrier against the intrusion of Ya'juj and Ma'juj. But, they still attacked through the pass in between the two mountains and which was closed by Dhul-Qarnain.
2. The word: زُبَرَ (zubar) in: زُبَرَ الْحَدِيدِ (zubar al-hadid - 96) is the plural of: زُبرہ (zubrah) which means sheets in the sense of pieces, fragments or ingots of iron which were to be used in the wall designed to block the pass, obviously a stronger device as compared to bricks and rocks.
3. The word: الصَّدَفَيْنِ (as-sadafain - 96) signifies the two sides of the mountain facing each other.
4. According to most commentators, the last word: قِطْرًا (qitr) in verse 96 means molten copper. There are some others who have explained 'qitr' as molten iron or pewter. (al-Qurtubi)
The expression: دَكَّاءَ (dakkaa'a - 98) used about the wall means what would crumble down to the level of the ground.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 91–93
Dhu’l-Qarnayn’s next expedition was towards the east of Iran. He reached a place where a most uncivilised people were living. The description that there was no barrier (shelter) between them and the sun means perhaps that they were nomads and were living on open ground instead of in properly constructed houses.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.