1960’s-1970’s YA mystery short story–people search a castle for a missing keep or room wherein may be hidden a fortune. Many people search. They decide to put a cloth in each window as they search; then they can see from the outside which window has no cloth and is therefore the hidden room. The story is possibly from one of the Alfred Hitchcock Presents Anthologies. It certainly had that feel. Although many others are searching this castle for a missing keep or a room in a keep, only one person finds it. There is some sort of secret entrance, I recall. When the finder discovers the entrance and room, another character (the bad guy) goes with him and murders him there and “hides” the secret entrance to the room again.
I recall (or believe I recall) a line at the end—part of the thoughts of the skeptical castle groundskeeper, I think– “there was no Norman keep.”