I am having trouble getting the 4th crystal. I have been all through this castle and once I get the madien she says to take her outside but when I try she won't go. I can't locate a monster either. I have the compass and the map but when I go into that big empty room that says it holds the monster nothing is there. I have been through this so many times and really don't know what I missing I have already beat 5 6 and 7 this is the only crystal I can't seem to figure out. PLEASE HELP

above that big empty room is a window and a weak floor, you have to throw a bomb into that area with the weak floor to open it up, you will notice the big empty room now has some light shining into it, if you take that maiden into the big empty room and into the light you will then have to fight the monster she turns into.