also in book 3, they explain why Castle black has no defenses - stakes, fences, walls, moats, or anything. This was done deliberately so that Castle Black is less indefensible from an attack from the SOUTH. The reason is so no Lord Commander can create his own fiefdom independent of the realm (not for long anyway). When/If the Nights Watch ever gets out of line (remember the ancient tales of the Lord commander that actually BEDDED a female wight, also the traitors of the watch that were buried up to their necks in ice...) then the Warden of the North, or any other Lord, can more easily attack Castle Black and restore the balance of power. No defenses from the south is meant as "encouragement" because of course,"the nights watch takes no part in wars of the realm". So in book 3 when the wildlings launch their attacks from north, AND the one by the Therns, from south of the wall, jon Snow has piled up barrels, old wagons and crates of grain etc. thrown up as a hasty defensive line in front of Castle Black, just to slow them down. Recall the wildlings make it to the stairs on the wall, but Snow sets the stairs on fire and burns a lot of them and Snow and his men halt the attack, fr the night...
