The perfect "closed setting" for a mystery story would definitely be something dark and something gloomy. It shouldn't be all rainbows and unicorns, or else the reader won't be interested or eager to find out more. Take The Hound of the Baskervilles, for an example. The setting right now is pretty creepy, the Baskerville Hall all big and scary, with no one else around. And, the fact that when we ended, Watson heard a woman crying, when trying to fall asleep in his bedroom. Without the dark setting, the mystery just wouldn't be as exciting or as thrilling as it should be. Setting, in my opinion, is the key to a great mystery novel.