The position of the LEver room has changed, now. That, or it just changes between seeds. Go past the Ballista guard, near to where the Caged stairwell is (the one you can't enter). Somewhere around there... there's a false wall to the left you can walk through, where the lever is hidden. Go along all the walls, I think you can see the lever room if you do
I may be wrong, but I'm not 100% the ballista guard always spawns. I think it's possible he's not there for you. I think...
The lever has been at the top of the prisons, toward the right end of the level, within another (unlocked, iirc??) room... at least for me so far.
It was suuuuper easy for me to walk right past the door to the interior room it's in, several times, because the doorway is kind of hidden.