A: It is Mance, not Nance
B: Castle Black doesn’t have the greatest defense on the south side because who in the right mind would want to attack the wall from the south? It doesn’t need to be impregnable.
C: There is a reason he is attacking the wall at Castle Black and it is because of that tunnel. I am sure the full reason will be addressed in the next episode. The nightswatch needs the tunnel to be able to range out in the north. It is as simple as that (and there is only one other way thru, the secret tunnel Bran went through)
I will agree with you on the mammoth point. Same with Alliser’s speech in the middle. Don’t talk just fight!