11 Five Have A Wonderful Time 1952

Whilst caravanning in the hills beside Faynights Castle, the Famous Five are eventually invited into the company of some extraordinary fair folk who are camping in the same same field, and meanwhile learn of the disappearances of two well-known Scientists who have presumably fled the country to sell some priceless Scientific secrets. The adventure really begins when the children spy a face at the window of a tower belonging to Faynights Castle, and following further investigation – find that the tower is not open to the public... Who was it, and how did they get into the tower when it was strictly out of bounds?