The language is a bit awkward but the story has promise. Sounds like he was the hero and the princess was the maid. Perhaps cursed by the demon lord.
However as
@Sep10trion said he could be the demon lord who was "banished" to this place.
On the other hand perhaps its both and he IS the demon lord but only because you become the "demon lord" by taking the previous ones place. This mirror girl will be his "princess" and a hero will come to rescue her, only to be trapped in the "castle" forever.