I just used the suggestion in the adventure that the players find a map of the tower that hints at treasure. I'm hoping the players of good alignment will make them stick around to help out. Eventually, they meet the G-Lord, who is the bbeg for my whole campaign...he's too powerful for them now (I altered him severely), so they will have to leave and deal with him later (or attempt and miserably fail). He of course has something to do with getting the map in their hands. He wants them to bring his FAKE item that was stolen (you said you didn't want spoilers, so there you go!). Yes, that's right, it's FAKE. Why the heck would a powerful creature like this let a few weak agents of RHoD steal it from under his nose? Nope, he's got a fake. The PCs only THINK they have a bargaining chip.
If they decide they want to leave after the tower because they aren't attached to this area, well then...mr. bbeg starts sending his generals and other minions after them. Eventually, they'll stick around. I dunno if this helps you or not, but that's how I plan to hook my pcs, which are currently 3rd level.