about jon snow's suggestion to block the tunnel, i think the reason for that is because the Wall itself is already a very good barrier (quite difficult to scale, otherwise the wildlings would have already done it en masse.) check out season 3 episode 6 - "The Climb"
the tunnel, while good for funneling the attackers, might still possibly be overrun with enough force and/or weak defenders. mind you, the night's watch isn't exactly westeros' most elite. it's possible that it might be easier to pick on those struggling wall climbers than to face them head-on at the tunnel.