The sally, an unexpected charge by the besieged could often turn the tide of battle. You will notice that the defenders can exit or sally out of the defenses by going into the ditch and then exiting just beneath the old Norman walls.By using the outer ditch the sally would end up outside the outer defenses. This was dangerous but it often slowed if not destroyed the besieging army's morale. Such a move could also be done in conjunction with a relief force. This happened historically during one of the sieges of Basing House.