in the castle throne room you talk to one of your ministers to arrange a dpolomatic visit to this capital town of the north, but before you go there, you go to a town close to this capital north town, search this town indoors for a uniform, after you get it, then go to capital town of the north and meet its leader after you finsh meeting the leader, then from the leader room go to the left side path continue all the way left and you will see some people enter a room in the north
this room will lead you to the mine
also from the leader room if you go to the right then you can save and add a magican girl to your party,
good luck