I think it's more likely RJ did this to further justify the Asha'man giving up the Black Tower. That, and those walls do have an ominous purpose: it's not to keep people out, it's meant to keep people in after the dreamspike was put in place and Travelling is no longer an option. Obviously the bonded wives, the families, the artisans and the few lightsiders still not turned the Shadow either means to use as hostages/bait or simply to slaughter them all. What would harm the Light more during TG? Would it be having the Asha'man pulling a Perrin and obsessing with rescuing their loved ones, or would it be to bear the pain of losing all their loved ones? Or would it be to play the first as long as possible then play the second option? The Shadow's answer to that question will likely determine their fate. If it's a plan in which Moridin "Betrayer of Hope" is involved, it doesn't bode well. It's close to the strategy he followed with Manetheren, and it's not all that different in essence to what he did to Lews Therin by clearing his mind after the Kinslaying. Targeting the loved ones seems like a favourite strategies of Ishamael. Again in TGS he instructed Graendal to make Rand feel loss and pains of heart... It never boded well for Elayne that she was surrounded by agents of Moridin either - Moghedien has long known of her relationship with Rand. That Elayne got pregnant, and Moridin/Moghedien wouldn't have doubted long from whom (but remember Shiaine's sudden alarm when she doubted an instant Hanlon could be the father), only makes Elayne a better target. All this makes me think the Shadow doesn't intend to strike during the Merrilor meeting, but timed the attack on Caemlyn for when the Light will scatter back to... wherever each group will go afterward. Moridin wants Elayne, I'm pretty sure. Moghedien hates her, Cyndane hates her or will hate her badly when/if she learn(ed)s she's carrying Rand's babies (and not only that, but "owning" Elayne/the babies would be a solid start toward a great revenge over Rand she's not allowed to kill), Demandred has once lusted after LTT's wife... not doubt because she was his wife. Elayne is meant to be a reward for one of those three, if Moridin manages to capture her during the battle of Caemlyn. I suspect in the epilogue we've already seen Cyndane at work trying to win that reward.

I don't have an opinion if the rocks themselves have special properties. I don't dislike the option they may extend the powers of Shaidar Haran in that location, so far away from SG. That could be really bad, a minor Shai'tan "avatar" sitting right in the middle of Andor.