Here's what some NPCs would suggest:
Approximately 48 hours after your palace experience, you travel on foot (or horse or griffon) to somewhere just beyond the edge of the no man's land. You choose a point of attack, and an agreed upon rendezvous on the other side of the wall.
Then, rather then depending on being able to see eachother, you depend on coordination. A mage of some sort casts invisibility on you, and you begin moving to the agreed upon spot, each with ropes and grapples. You will be unable to see eachother, so timing is of the essence. After crossing the no man's land (as quietly as possible) you all grapple the wall at pre-chosen places, and begin a fast ascent. The grapples (and ropes) may be noticed, so you'll need to ascend with the utmost speed.
[alternatively, you may wish to stagger your ascents so that some characters can provide a distraction while others climb]
Once at the top of the wall, you'll dispatch any opponents you may encounter as quickly as possible, and then move to one of the guard stations/towers as fast as possible. If you've encountered resistence, you'll be visible, so speed and silence are extraordinarily important. Moving to one of the guard posts should aid you, as just trying to cross the inner no man's land will attract undue attention. You'll descend where the draconids do, and move across the inner no man's land imitating them as well as is possible. Xath should be able to help a little, here.