I remember reading in the rules that a gold cube can be used as a substitute for any other resource cube for any purpose. I can no longer find that in my rulebook. Is it there, or was I dreaming?
It is not there.
Gold is a resource like any other so if a building or other use (market, building a section of the castle) requires a cube of any kind, gold is a kind and can be used. But it's not wild.