Thanks, Neil. It is a good thought that, just because the PCs are coming in from downstairs, it doesn't mean the big battle NEEDS to take place there, if somewhere else is more advantageous for Aisa or more interesting. The witches have already been alerted to the PCs presence, because one of the Oothi spider swarms is in the cave, and the witches are telepathically connected to her/it.
But it may be that Aisa sees some advantage to moving upstairs before the PCs come, perhaps to fully alert and organize the upstairs troops. Perhaps she thinks the bloodknight and ex-paladin will soften up the PCs (assuming the PCs don't ally with the ex-paladin). I suppose I could also imagine her wanting to have Hetna take on the PCs first. (I still can't imagine why she would think it would be advantageous to feed on Hetna rather than having Hetna as an ally in the final fight -- unless I guess she just views it as an easy opportunity to get rid of her sister).
Anyhow, it is interesting to consider that it can be a re-arranged sequence rather than a battle royal, although a battle royal is probably the bad guys' smartest move.
Any thoughts, building on Neil's idea, of how Aisa might want to throw foes at the PCs, or where would be most interesting to stage the battles?