This seems to be one of Blyton's earlier works and appears to have been written prior to the first of her Famous Five books. This story involves three children: Pam, Peter, and Brock, and they are going on a holiday to a sea side village. At the village there is a castle at the top of a bluff, but it is not a real castle, just a modern building that has been built by an eccentric millionaire who died years ago and whoever inherited it never took any interest in it.
Being kids they decide to go and explore the castle, and being kids they find a way inside. What they don't realise is that some crooks have also found their way into the castle and are using as a base for their nefarious deeds. As can be expected from an Enid Blyton mystery novel the kids end up tricking the crooks and capturing them, then the police then turn up, arrest them, and the children are told how wonderful they are.
As I said this is an earlier work, so the story is shorter and fairly basic. The only strange thing that goes on are the mysterious lights in the castle, and of course the villagers are telling the children to stay away (though for some reason their guardians are not particularly concerned). Also, there is no dog in this adventure, though the use of the dog as a part of the group seemed to only work in the Famous Five. Timmy, as I have mentioned, is one mean mutt, where as in most of the other stories the dog simply tags along and never actually helps solve the mystery.
This is not one of her better books, but was probably written in her transition from fairy tales to mystery stories and other forms of children's books. I note that there are also a number of stories set in a school, and a number set in a circus (though I do not know exactly when they were written). In the same way that the Faraway Tree stories were probably (in my opinion) the best of her fairy tales, the Famous Five were the best of her mystery stories. Since I am reading through all of the Enid Blyton books that I have (and also trying to get my hands on the rest of the Secret Seven books, if that is at all possible – doh, I knew I forgot to look for something when I was in London), this one does fall on my list.