I do need to foresee rappelling as being particularly effective given the fact that if anything goes wrong, at all, there is no credible line of retreat, let alone if the rope in some way fails. A character could easily become stuck in transit or the obvious issue of the rope breaking. 

Is there anything from preventing the group from merely approaching the entrance to the area, then hugging the wall to prevent line of sight? Seems more reasonable and less needlessly risky.