Yes I know it is a fantasy book. Why build a wall using metal? Aren't there any better building materials like for example reinforced concrete, or even stone? Also how do you even build a metal wall? Do you pour is straight in place, or weld metal plates (that doesn't make any sense, because the plates would be too thin). And the final question - if all the metal is used for the wall do they have any cutlery, belt buckles, hinges or are they too expansive for the common folk?