The Castle of Adventure by Enid Blyton

Why is everyone so afraid of the castle on the hill, and what dark secrets lurk inside its walls?

Philip, Dinah, Lucy-Ann, Jack and Kiki the parrot are on holiday in the countryside, staying on the side of Castle Hill. When flashing lights are seen in a distant tower, they decide to investigate - and discover a very sinister plot concealed within its hidden rooms and gloomy underground passages.