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Can you give more details? Like how is it cursed? Is it an official site, you are using? Did the players roll a 4 or 6 on the Stronghold flaw charts? Or is it maybe a roll of 52-65 on the random castle creation chart?

That roll of 52-65 has no official way of being removed. But as with all curses, there is probably a reason for it, so you could just come up with a way of WHY the place was cursed and undo that.

Examples:
Why: Two important families planned a wedding, but there was betrayal and murder.
Add clues about this: like castle weeps blood during moonlight. Players suffers nightmares of being murdered. Spectral figures may reenact the act during the night if followed.
To get rid off: Find and properly bury the hidden remains of the massacred family. Find relatives, make them attone, forgive, visit the place, etc.

Why: The former ruler still wants to come back from the dead to rule the place.
Clues: Portraits change to resemble builder. Sensations of being watched. Dreams offering power for aiding the builder in his quest. Players may become more power hungry themselves.
Lifting it: Perform a ritual that drains the curse by making him whole again as a Death Knight (the ends the curse, but they might want to remove him too after that). Or find all his scattered remains, and perform a purge undead spell on each part. Find the parts and use some kind of speak with the dead ritual and make him see that his life brought nothing but pain.

Why: Perhaps a demon, nature spirit, or the like, was bound to the foundation of the place. And now it after the castle has been neglected, it has regained some power to break free, but not yet done so.
Clues: Strange whispers in unknown laguages, strange magical effects, etc..
Lifting it: Find the original binding rituals, find the bindings in the castle, an unbinding ritual could be an epic multistage scene, if you do things wrong maybe parts of the castle starts to crumble. Alternatively, maybe change the imprisonment to a partnership?
... etc ...

• Have the PCs discovered all the details about what happened and why it is cursed? This will help them get rid of it. Perhaps they could also find a damaged diary, etc.
• The might need to do some research to find more about this sorcerer, in a nearby village perhaps?
• The might need some things for the ritual "something not touched by the red mist", "the armor of a rust brother", etc
• They might find some remains of those that died. If they bury them with som of their belongings it might aid them, if they steal them, this will make it harder (may turn against them in a final battle, or so)
• The sorcerer might want an item in payment, perhaps an item located at the Stronghold? Or they might just owe him a favor..
• If the Stronghold has prisons, torture chambers, execution racks, etc.. the players could be given the dilemma of destroying them to gain a boon towards lifting the curse.
• When the curse breaks, perhaps most of the spirits find rest, but the evil tyrant might become stronger, able to take form in some final battle.