Great suggestions, and from what was mentioned above, I think I will flesh out the castle more, drop a couple of more hints, but the wizard and prince will have a head start on their plans (raise army and overtake the kingdom w/ the magic item they stole from PC's).
How 'bout using a cutscene as a clue? Maybe this way the players will not spend alot of time in the castle. Something like this:
A desolate, lit candle pierces the darkness. It stands atop a rustic table and reveals a hooded figure; the rest of the room is pitch black. The flame suddenly begins to flicker violently as the door opens and someone enters.
"Sir, the plan is in motion, " says the human guard, dressed in expensive chain mail. "The wizard has the mithrall, and will be here by sundown."
"Excellent," replies the hooded man, and stands up instinctively.
"Assemble your men, for the time is near. When the wizard arrives, inform him that we are ready for the next stage."
The guard, afraid to ask the next question, musters enough courage to say, "And your wife, sir?"
The hooded man turns to face the guard. "She is not a threat," he utters. "By sundown... she will be dead."
He exits the dark room to find his army lined down the hallway. In a thunderous chorus, they proclaim, "All hail Prince Andar, future fuler of Zoa!"
Maybe I can have the cutscene after they explore the castle a little bit more.
Thoughts?
-PJ