From what I remember, Humans were just starting space travel and got into a space war with a warrior race and got beaten badly. The warrior race were so disgusted with how poorly the humans fought they put the soldiers on a huge prison ship (to them this was the biggest insult).
The prison population was kept in a single, huge very tall room. Up and down the walls of the room there were small alcoves that would periodically dispense food, but not enough for all prisoners so they had to fight each-other off to get rations.
The gravity in the room varied so prisoners higher up could fly with some effort; but it got harder and harder the lower down the wall one went. Gravity on the floor was normal. The weaker a prisoner got the lower down the wall he would sink until he was stuck on the floor. There were no food dispensers on the floor level so all there was to eat was each other. The whole system was designed to humiliate the pitiful warriors and cause them to fight each other for food and to ultimately resort to cannibalism.