﴿ فَمَا ٱسْطَٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَٰعُوا۟ لَهُۥ نَقْبًۭا ﴾

“And thus [the rampart was built, and] their enemies were unable to scale it, and neither were they able to pierce it.”

And so they Gog and Magog were not able to scale it to climb up its length because of its great height and smoothness nor could they pierce it because of its firmness and thickness.