What is the secret of the old castle on the hill, and why are the locals so afraid of it? When flashing lights are seen in a distant tower, Philip, Dinah, Lucy-Ann and Jack decide to investigate - and discover a very sinister plot concealed within its hidden rooms and gloomy underground passages. Other titles in the Adventure series: The Island of Adventure The Valley of Adventure The Sea of Adventure The Mountain of Adventure The Ship of Adventure The Circus of Adventure The River of Adventure