Is there anything that explains where they get their minerals from? I suppose all the mud comes from outside, and maybe the lime and whatever else they need to make mortor, but for copper I would think you'd need a mine of some sort, unless they been canabalizing the spire itself for the last two centuries.
Also, what about the iron/steel? Iron seems to rot so quickly, and yet they have so much "copper clad iron". They must have some source for, and more importantly, some way to forge it without it rotting, to give them time to cover it with copper.