In AD&D there is no way to climb a wall unless you are a thief and have the Climb Walls ability. Casandar is going to need a rope or something and be hauled up.
Let's work out where everyone is:
Sin-Nasir was on the dais, on a solid part that didn't collapse.
Casandar is down a pit under a part of the dais that did collapse.
Thalia is mid-way on the wall opposite the door, about one step away from the wall.
Aelfwine, where were you when you took a step towards the dais and tripped an acid globe?
Here is the room. I'll update it as we go.