The NPC side didn't directly pad too many precious materials on the Holy City. After all, given the size of the city, even if the NPCs had deep resources, they would not be able to bear such enormous consumption required to line an entire city with materials.
However, even though it was unrealistic to line the underground with metal or similar materials, they couldn't leave it as it was. If they encountered monster groups that like to burrow, wouldn't the defense of the Holy City have a major loophole? Thus, after conducting certain experiments, the NPCs had decided to strengthen the ground using Magic Arrays.
As a result, after those NPCs reinforced the underground soil, its hardness was now comparable to steel.
With such common attacks causing immense destruction, one could infer that the entity that had suddenly attacked Zhou Tian previously was incredibly powerful.
