The cover of my 1960's era paperback of Death in the Castle. described the novel as "chilling and thrilling". It isn't.
Sir Richard and Lady Mary need to sell the thousand year old castle which has been in the family for hundreds of years. Sir Richard has no heir to pass the castle on to, and if he doesn't sell the English government will turn it into a prison or a powerplant. John Blayne, an American, wishes to buy the castle, but he wants to take the castle apart piece by piece and bring it to America and make it into a museum. Sir Richard does not know this and this becomes a source of conflict.
The maid, Kate, is more than she seems to be and while the reader is given enough clues as to her identity, most of the other characters have no idea.
There is supposedly some sort of a mystery here regarding the sale and the mysterious "they" who also inhabit the castle, but to be perfectly honest the story is not very interesting, there is no mystery, there is nothing thrilling or chilling and the novel is little more than the barest thread of a story about a couple not really willing to sell and a buyer who wants to be honest and respectful.
This isn't very critical, but the whole book and uneventful storyline just bored me. It's a short novel, but there is no substance.