As long as you possess a Prison, there is a chance, after a combat in which you are victorious; that you will capture any enemy Units defeated in the fight (to include those which flee). (You cannot Capture Prisoners as a result of the Harass Travelers action.)
The odds of capturing a given Unit are as follows:
- Having a Prison nets you a 70% chance.
- Each Renovation level beyond the first adds +15%
- For each level, of the enemy Unit: -4%
- Per level of either Tactics or Outmaneuver of the enemy Unit: -3% (Highest skill rank is used.)
It should be noted that you can capture Leaders, Extras, and even Monstrous Units if you have a Prison (except for Minotaurs, Dragons and Giants. They just don't fit.)
Leaders and Extras each take up a full prison slot. Leaders captured from the Faction Forces can be ransomed or even extorted. You will receive specific information if you capture one of these.
Extras can be interrogated or converted normally - except that if you convert an Extra, it does not change which Extra it is: you simply gain the Extra you had captured in the first place.