This particular room you described is where the bosses stay - in one of the battlefields dungeons/levels.
You know, you get throughout the game several battlefield keys for different BF levels/dungeons.
It occured to me, that sometimes bossess are not in one level, but in the next.
I think I even once had almost all bossess in the last room (similar to what you described) in the third BF dungeon/level (I mean the third BF key there).
