Return to the entrance (of the castle) and take note of the staircase that appeared on the left side going up to the top balcony as well. A new entrance is in-between the stair case and the first arch you entered on the right. This leads to the courtyard in the balcony area, and during your trip, look for some chest on the chandelier high above. Continue across the balcony to the upper archway, and descend the steps into a whole new area.
Ride the elevator in the center of the room down to find yourself in a room with Odins electrified sword chained to the back wall.