Castiel found out really suprising and interesting thing.
You surely heard that angel wings are most sensitive part of his/hers body. And Dean was often using it against Cas, since Dean was demon. And it was not pleasant for Cas.
But Castiel found payback.
Thanks to his angel being, he can see true form of a demon, what includes his horns.
And horns are as much sensitve as wings.
You never heard Dean-mon beg, Cas did. And he will always throw it into Dean's face, because who's begging little guy now?
