فَمَا ٱسْطَـٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَـٰعُوا۟ لَهُۥ نَقْبًا Qur’an Al-Kahf (18:97)Fam a is ta AAoo an ya th haroohu wam a ista ta AAoo lahu naqb a n
Lit., "they".
The iron wall and gates and towers were sufficiently high to prevent their being scaled and sufficiently strong with welded metal to resist any attempt to dig through them.