Sean-H-

Right on this part...i used a walkthrough from this website which helped me....it said use the squares as numbers so:
1 2 3
4 5 6
7 8 9

And then put 4 to 5, 7 to 4, 8 to 7, 9 to 8, 6 to 9, 3 to 6, 2 to 3, 1 to 2, 4 to 1, 7 to 4,
8 to 7, 9 to 8

If this doesnt work, it would best trying the walkthrough from this website...because thats what I used for this part, after you done this, and put in the missing piece that you should have picked up earlier in the mission with ashley, the door, should open,and then close behind you, once it closes get the serpents head or some kind of statue from the treasure chest,once you get the treasure chest, the door should then open, but the statues of them knights come alive, so you have to avoid them, because she cannot fight...once you make your way back through were you came from, avoiding all the knights, go into that room where you needed to insert something, then the door should open, and then follow the path till you reach leon again.