The young prince is heir to the throne. The evil witch comes to court pretending to be (or perhaps really is) a noblewoman. Her plan: Marry the Prince, bump off the King, then when she is Queen, she begins to systematically assume more and more control over the kingdom.
I suppose this could become a bit of a humiliated husband scenario, too.
An alternative plan would be to marry the King, knowing the Prince has the hots for her, and play them against each other. There could certainly be a role for a dominant male who is no friend to the royal family, but is perhaps a Baron who has only given nominal support for his king, and would make a perfect lover for the Queen.