"Castle of Adventure" was the second of Blyton's "Adventure" series books, and was published in 1946. Unlike some others in the series it kept its title in the U.S. edition.
The book reintroduces Jack (the bird lover) and his sister Lucy-Ann (a bit on the timid side). They are orphans who are informally adopted by Mrs. Mannering because of their friendship with Mrs. Mannering's children and because of her great fondness for them. Mrs. Mannering's children are Philip (the animal whisperer) and Dinah (squeamish about animals, but level headed and independent). The cast is rounded out by Kiki, Jack's pet parrot and the comic relief in the books. The children's ages range from 14 for Jack and Philip to 12 for Dinah and 11 for Lucy-Ann.
In each book the two sets of siblings find themselves on school vacation with Mrs. Mannering, (and by this book also with Bill Smugs - Mrs. Mannering's friend and a member of an unspecified British secret service). Each book starts out with the family going on a simple, relaxing holiday, (here it's Easter in Scotland), but mystery, suspense and skullduggery always rears its head about three chapters in. The books feature islands, castles, underground rivers, subterranean tunnels, spies, smugglers, assassins, and other neer do wells.
In "Castle of Adventure", the children are staying at a cottage in the Highlands. From there they can see an abandoned castle high above at the end of an inaccessible road. After meeting a local girl and learning of a secret way to the castle all sorts of mysteries are uncovered and adventures undertaken.
This book recaps all of the characters, their relationships to each other, their various personalities and the group dynamics, (Philip teases Dinah, Lucy-Ann dotes on brother Jack, and so on), that will be consistent throughout the series. It's not necessary to have read book one, ("Island of Adventure"), or to have followed the books chronologically, but it probably would be helpful since the first book does establish all of the basics. (That said, each later book does rather efficiently set the basics out again, in the first chapters, for new readers.)
This is a stronger book from a girl's point of view, since the two girls are now much more involved in the most exciting parts than in the earlier book. Perhaps the strongest note, apart from the whiz bang adventure, is that apart from some teasing and the like all of the children trust and admire each other, and the siblings are loyal to each other and their friends. There is none of the incessant sibling conflict that is popular in many current books. There is no moralizing or preaching, but there is a powerful undercurrent of courage, bravery and doing one's duty that is always in the background.
Blyton was criticized by "serious" reviewers for using short declaratory sentences and an undemanding vocabulary. Her response was that she did not listen to any critic who was over 12 years old. That said, these are not light weight or simpleminded books. The characters have charm and individual personalities. There is no pretense of grand literary accomplishment, but these are exhilarating and suspenseful adventure tales, well told. It is a testament to their quality that most of the reviews at this site have been written by people who read the books in the childhoods and still remember them vividly.
And, these books do stand the test of time. Caves, tunnels, floods, deep holes, secret passages, hidden doors, underground rivers, abandoned buildings, creepy noises, mysterious lights and sounds - all of the standards of tween adventure - start here, in the hands of a writer who knew how to tell a gripping story.
These books were hard to find in the U.S. until reprints began to appear. I'm delighted to see that we now have Kindle versions, which makes it easy and relatively inexpensive to try one. If you have a tween or even younger reader who likes gung-ho adventure and imaginative larking about, then one of these might be worth a try.