[Lox]

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If i recall correctly, the locked door to the lower level is near where you found the box. When you come into the dungeon and are in the big entrance room, the puzzle box is on the middle level (you have to walk across a ramp/bridge to get there). On the lower levels are three doors. Behind the doors is the main part of the dungeon and somewhere down there is the door I think you are looking for.
Of course, I never did explore the entire dungeon. I was fairly low level so I just got the box and got out. I do recall coming across a door I couldn't open (lock of 75 or something). I think that is the door the key opens.