That weird eye familia must have a transmutation spell that allows turning rock into orichalcum and adamantite. Nothing else can explain how there has to be millions of tons of "rare" metal in that artificial dungeon. If you think about it, even if all the walls were reinforced by only steel, it would still be a significant amount even for an industrial society. It can't be half a millimeter of metal, after all, because then pretty much any hard thing would be able to break it with a bit of effort. I'd say at least four millimeters is the minimum to make a real difference when we are talking about high level adventurers, who are multiple times stronger than normal humans.