You need to currently own five million gold (or more) to walk up to the key that, for the record, is not a gold key. It may be gold (or it's brown, I don't really remember) but it does not open any gold doors.
After you've acquired the key, move all your gold to the Treasury (you can only do this after you've been crowned King or Queen) or simply spend all those millions on weapons and clothes and what have you. Then you can reenter the Sanctuary and open the chest right below where you got the key. I don't know if it's random like most chest, but I got the Dragonstomper .48 (and some other things, if I remember correctly) from opening it.
EDIT: Oh noes, TWO NINJAS! D: