At a certain level (4 or 5 ?) a city can evolve to a Prison.
The description given is that it is supposed to decrease by 10% the unrest IN ALL THE CITIES.
Well, it doesn't seems to work : Having some unrest issues (too many cities) I've turn one of them into a prison, hopping it would decrease a bit the unrest, but it still a 100%.
And when I look at the detail of this unrest, it's writen nowhere in this detail that I've got a 10% decrease thanks to the Prison.
So I guess it's a bug.
PS : I'm playing the LH scenario (I can't tell if it's only in this scenario or in any new game that the issue occures).