1st - Detention facility where u can put max limit 5 workers in it. when worker in jail he doesnt cost any gold per turn.
2nd - Concentration camps - where we can place captured workers (ex soldiers,setlers, workers if in the future we will able to capture enemy inf turn them to pow-worker) each pow could be just like a worker but will not have effect in jails instead in CC they could make 2 shields and 1 gold to nearest city and maybe 1 unhapiness if country sensitive to war wearness.
What's next, torturing captured workers for information or executing them?
Maybe a random event of workers liberating their camp, with probability based on number of inmates.
Other countries, angry over treatment of POW, get 10% attack bonus?