The Wall says "Challenges: Put one Night's Watch character into play from your hand, Knelt as a defender, during any challenge declared against you. (Limit once per challenge.) Return that character to your hand at the end of the phase if it is still in play."
Question 1: Can you use this ability if you have no other characters who can defend in a challenge.
Question 2: Can you put a Night's Watch character into play as a defender, using the wall, during a challenge in which the character does not have the appropriate icon for that challenge type.
Question 3: Can you use the wall to put a character into play as a defender if you can't "declare" defenders. (or does putting the character into play as a defender count as "declaring" a defender.)