One of the most popular schemes employed by mystery writers can be described as "A Closed Circle of Suspects." This fairly well describes the story I'm working on right now. The setting for my story is a Health Spa Resort in a foreign country. But this isn't a truly closed circle because the characters can come and go if they want to. They aren't captives. I'm fully expecting someone to level this charge at me at some point. But I've learned something important about this business of categorizing fiction, especially mystery fiction. It doesn't matter! There are no rules that can't be broken; this isn't a matter of rules anyway. At most this is simply a way of looking at a story and seeing its possibilities and limitations before you start writing.
The "Closed Circle of Suspects" category is surprisingly flexible, and successful stories certainly number in the hundreds, if not thousands. All you need is a setting that encloses a definite group of people so that, when a crime (usually murder) occurs, you know the perpetrator has to be one of that group. It would be cheating, I think, to bring in a long-lost heir to the victim's fortune in the last chapter. "Hi! I'm Malaria von Hatchet. Am I too late for Grandfather's funeral?" The suspects are well defined and don't usually number more than about eight. That's another reason my story is not a true closed circle. There are more than a hundred residents at the resort at any one time. They can't all be suspects.
A common Closed Circle mystery is the English Country House Mystery. Examples include The Mysterious Affair at Styles, The Mousetrap, both by Agatha Christie, and The Woman in White by Wilkie Collins. But it doesn't have to be English. How about Knives Out? It's in a country house, but it's set in the USA
A Closed Circle mystery can also be set in many places other than a country house.
A ship- Death on the Nile, The Woman in Cabin 10.
An Island - And then there were None, The Lost Island
A Plane - Death in the Clouds, The 12:30 from Croydon.
A Train - Murder on the Orient Express, Strangers on a Train
You get the idea. The advantage of mystery stories with closed circles is that the list of possible perps is limited. This gets one variable under control and simplifies things a bit. So the story is usually concerned with opportunity, (Who had access to the gun cabinet? Who left the dining room before dessert?) or motive (money, jealousy, fear, etc.)
The kind of mystery I most like to read is the police procedural. I enjoy reading them because they are so different from what I write, I don't connect them with my own work. If I did, I would be constantly comparing the book with my own. In police procedurals you almost always have a a huge area, like a city, in which to find and trap your killer. It takes the skill and training of a professional sleuth to deal with the possibilities.
Many cozies are, to some extent, Closed Circle stories, like the village of St. Mary Mead. Donna Andrews's bird-themed books are set in Yorktown, VA. Rita Mae Brown's Mrs.Murphy books are set in Crozet, VA. Crozet is such a small town, it almost qualifies as a Closed Circle.
So. Whereas my current WIP is not strictly a Closed Circle, the story does concentrate on one particular group of guests at the spa resort. I guess that makes it An Almost Closed Circle of Suspects.