There is a tip on one of the loading screens explaining that I can take certain people I meet prisoner. There is also an upgrade for my Stronghold that is required to do that.
What do I gain by restoring my dungeon and then imprisoning people? I'm trying to decide whether to upgrade that part, and when to upgrade. I'd appreciate a mostly spoiler-free version without story specifics, I'm mostly interested in the general advantages of building the dungeon and imprisoning people.