This forceful gathering, the "kneeling" of every nation 45:28, is particularly pictured through the story of Gog and Magog.
They were an ungodly people who terrorized those weaker than them and the Prophet-King Dhul Qarnayn managed to contain them behind a metal wall, of iron and copper, to protect his subjects. Rulers of all times built barriers and still do today, to protect their people from outside enemies and invasions. The prophet-king Dhul Qarnayn was no exception and as a sign of his noble character, he regarded it as an unconditional duty of his to protect his subjects
After having succesfuly done so, instead of pronouncing his own greatness as Kings normally do, this Noble and Pious King tells the people that all things will eventually crumble when their appointed time comes, as stated in the sura's beginning as well as often repeated elsewhere in the Quran, including his own monumental achievements which are in fact not really his, but the result of God's mercy upon him18:94-7"Shall we then pay you a tribute on condition that you should raise a barrier between us and them? He said: That in which my Lord has established me is better, therefore you only help me with workers, I will make a fortified barrier between you and them".
The whole sura concludes with a reminder of such principle which must be impressed in the mind of the God-conscious18:98"He said: This is a mercy from my Lord, but when the promise of my Lord comes to pass He will make it level with the ground".
. Since all things, even the most monumental constructions are eventually reduced to dust and levelled with the ground as stated by Dhul Qarnayn in the verses, not finding any trace of it today or its definite location agrees with Dhul Qarnayn's statement concerning its inevitable end."they whose labor is lost in this world's life and they think that they are well versed in skill of the work of hands. These are they who disbelieve in the communications of their Lord and His meeting, so their deeds become null"
The more profound message is that escaping that rampart, something that is bound to happen as per Dhul Qarnayn's words given the temporary nature of the structure, does not mean that one can escape God's justice, including the marauding barbaric tribes of Gog and Magog, hence the following statement from God about the Day of Judgement
In this recurrent depiction of Judgement day, the masses are gathered regardless of their differences, crawling over one another to answer the call of the trumpet to be judged by the Lord, such that it will look like wave upon wave. At that same time that the towns who otherwise would never return, will be brought back from their death to answer to the call, the Gog and Magog will also be raised and let loose to descend from every elevation since all these natural places of refuge once particularly used by these plunderers will be destroyed18:99"And We will leave some of them on that day surge against others (like waves); and the trumpet will be blown, so We will gather them in gathering".
.20:105-7,21:95-7"And it is binding on a town which We destroy that they shall not return. Until when Gog and Magog are let loose and they shall descend from every elevated place. And the true promise shall draw nigh, then lo! the eyes of those who disbelieved shall be fixedly open: O woe to us! surely we were in a state of heedlessness as to this; nay, we were unjust"
A general state of upheaval is being described in both these passages from suras 18 and 21, with Gog and Magog, a people once known for being nomadic, uncivilized and ungodly putting their lifestyle aside, coming irresistibly together with all of humanity, surging like waves upon waves towards their final reckoning.
Nothing is said in sura 21 about the necessity for a wall to be destroyed for them to come out since it obviously is talking of judgement day and their forceful gathering with all humanity, ie the previously destroyed towns have now returned, and in sura 18 no connection exists between the surging of Gog and Magog, along with all of humanity to answer the call of judgement (which is God's statement), and the temporary nature of the wall spoken of by Dhul Qarnayn to his direct addressees.
The mention of Gog and Magog and the towns in the context of resurrection and judgement clearly is meant at picturing the fact that people will be brought back to God from wherever they might be, whether they lead a sedentary or nomadic and untamed lifestyle.
As a side note, after the Gog and Magog's rise, the "drawing nigh" of God's true promise made to the disbelievers spoken of in 21:97 is in reference to the now inevitable prospect of punishement they were promised in this world and to which they were heedless, their soon hurling in hell spoken of in the following verse after their judgement 21:98, as they look on with terror inside the now opened gates of hellfire that is brought near to those concerned 18:100,26:91,79:36.