I'm still a little confused about how this rooms works, do you drop only prisioners for the beasts to fight and gain experience? Can they instead fight among themselves? If so, why would you waste a prisioner in the arena? Can your other creatures fight in the arena against beasts or other creatures? When it says toggle to save the fallen minions does it mean that prisioners will also be spared?
By the way, great work in the room, the arena looks splendid.
Ok, let me try and break it down for you so it's a little easier to understand...
The Beastmaster is attracted by the Arena, his work role involves him fetching 2 beasts and making them fight in the arena to train them up. In this situation, when one of the beasts loses.. it will be KO'd and sent back to it's lair whilst the winner stays in the arena to keep fighting.
Intelligent units can also be dropped in the arena by the player (but are not fetched by the Beastmaster), they train in exactly the same way, fighting other units in the arena, but can be executed (if the switch is set to kill rather than KO)
If the switch is set to KO mode, intelligent units will be KO'd and sent back to their lairs when they lose a fight, if it is set to execute mode, the winning unit will be allowed to execute the loser.
Executing a unit (whether that is one of your own intelligent units or a prisoner unit) gives a large chunk of exp to the units currently training in the arena.
Prisoners work the same as your own intelligent units in terms of dying/being KO'd (Beasts cannot be captured to use as prisoners, they just die in combat.)