فَمَا ٱسْطَـٰعُوٓا۟ أَن يَظْهَرُوهُ وَمَا ٱسْتَطَـٰعُوا۟ لَهُۥ نَقْبًا Qur’an Al-Kahf (18:97)Fama istaAAoo an yathharoohu wama istataAAoo lahu naqban
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.