You have descended into lower levels using the drop chute, each PC hanging onto a downward-moving handle and linked to one another by a safety rope. Invisible Ibben is on the lowest rung, followed by the others in marching order. The retainers stayed put on the ‘top deck’ for now. Over a descent of approximately 170 feet, you passed by three levels. The first was blocked by a closed doorway, from the second you saw a huge open area, and the third is the botanical garden.
Further descent is blocked from below. Peering down, Ibben sees that the bottom of the shaft ends in a garbage heap (any access to levels further down appear to be buried under refuse).