When you cage invaders, they just become an item like any other. And you get rid of them by either letting them out and killing them or destroying the cage.
To me, it gets tedious to "process" a lot of cages manually, plus it feels historically incorrect. It feels weird to have to destroy the entire cage in order to get rid of a prisoner inside it.
So I'd like more options:
1: Execute.
The cage should have an option to execute prisoner, so with just one click he gets stabbed through the cage and stripped of his stuff. I guess the chief of the guard does the executing.
2: Free.
Historically, prisoners were sometimes led free on a oath they would cease hostilities, or simply sold off. On this option, you ask the prisioner to swear to cease histilities in return for his freedom. If he accepts, the prisoner would get stripped naked, set free from the cage, and then walk straight of the map and home.
IDK if it an be implemented that they keep their oath, but you could for instance make it so that if freed prisoners reach their home site, there is a % chance that they stop attacking you for five years. It's fine by me if they show up again, it would motivate you even more to kill them.
There is a lot of possibilities here, ransoms were a big deal. You could go overboard and have a messenger go and ask for some stuff in return for you freeing the prisioners. Or some emissary could show up and give you stuff in return for freeing the prisoners.
But I would be happy with a very simply solution that just rids you of them.
All the old solutions would off course still be possible.
3: Slavery. I personally think it is too dark for the game, but if the developers feel like it, it would make sense to keep them as slaves or servants of nobles or something. IDK how it could work, but it would be neat to have some slaves from a former invasion.