There's supposed to be a series of jumps to get you up there, but I've only ever spoken with one person who did it, and she could never replicate it.
She said there was a treasure chest with unremarkable loot and a couple of barrels.
I've devoted many hours to trying to get this one, as have plenty others.
None have been able, to my knowledge.
Did you notice that you can see windows in the towers from the inside in both Jade's and Quaalus' places which aren't there from the outside?
Welcome to the Lair, by the way!