Built according to your doctrine of 'not being excessively nice to the prisoners', this heightened-security prison can fit up to 25 hostages, while still being considerably cheap. It features a 'fun room' (+5 Workshop space) for changing riffraff into Slave Halfwits. However, Dungeon's special status means that only a single one can be constructed in each Hideout, plus it requires a Cultural facility to be present.
Warning: presence of any Dungeons will undoubtedly draw much ire from our enemies, which means they will launch Crackdowns against our Hideouts more often... I understand it was your plan all along?