Hmm how about this then, one cannot gain the ++ class via this castle but instead it offers them a peek of a boss version of each ++ class to get a taste of what they can do. There is a main boss demon outside of the ++ bosses but in order to unlock his room you must first defeat all 12 bosses in each of their wings.
I suggest as a reward for beating the ++ class that reflects the one your currently using I.E. Dragoon / Dragooness against the ++ Dragooness boss or Dark Knight / Black Knight against the ++ Black Knight that character would gain a fragment of their weapon this in turn would allow the weapon to be reforged as that characters best class weapon.
as for the main Demon boss how about it is an underline of Archimus
and holds the key which would allow you to face him and thus unlock the 2 ++ classes as the game already is?
doing it this way you would be able to test out the ++ class in a fight / get a better weapon / and extend the current ++ class quest a little more ^_^
My I also suggest the final boss being