The book ends, have no fear the twists won’t be shared, using a very clever device. The Countess writes a play – the Haunted Hotel – and it is through that we learn, or perhaps learn, the truth of what went on in the palace.
All of the necessary characters draw close to the hotel so the ending can be complete but the reader is let in on the full hand of cards when most of the characters are not. In that respect it is a satisfying end.
If you were looking to find fault, and in many ways it is perhaps just nit picking, then you might argue that the story is perhaps a bit stretched. The most interesting parts happen in the past and maybe this would have been a different book had the action focused on those moments rather than to tell them briefly towards the end.
Still in terms of the book delivering what you want, which is a creepy chiller to mirror the dark nights then this delivers.