The puzzle with the deben was much earlier in the game, Karen, you've already passed that point.
To pick the correct shield from the armory:
Read all pages of Harker's journal.
Read all pages of Myths & Legends book.
Re-check the tapestries in the hall which depict a shield.
Click on the shields in the armory and you'll see them in close-up. It's the middle shield that you need.
When you click on the moons in the centre of the middle shield, do they move?
If so:
Click the 2 moons to face each other and the 2 ends overlap.
Click the outside ring that goes clockwise 12 times. Click the middle ring that turns counterclockwise with the darts 3 times.
And that should do it.
Also pick up the coat of mail and the crossbow which are at the left side of the armory, if you haven't already.