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.