Directions to the Cannon Room from the Quad (orange line on map below):
When you are facing the Quad, walk up the RIGHT set of stairs and along the walkway until you come to glass doors on your RIGHT.
Go through the two sets of glass doors and make an immediate LEFT (a security guard/desk will be on your right).
You want to go LEFT and walk through a green set of doors. Keep following the hallway and the Cannon Room will be on your LEFT in a short time. If you have come to a black and white checkered walkway, you have gone too far.