The Secret of Moon Castle (The Secret Series, Book 5)
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Prince Paul's family are visiting England and wish to rent a castle to stay in for several weeks, and Mrs. Arnold has been asked to help locate a suitable castle, knowing England far better than Paul's parents. The Arnolds are invited to spend some time in the castle with them, too, and are soon puzzled by strange events happening in the castle: odd noises that occur, eyes in portraits that shine mysteriously, lights that shine in tower rooms, despite being locked tight. It seems that there are unknown people who do not want them to stay in the castle.
On occasion, a strange tingling effect is felt by them, and strange coloured lights seem to rise from the ground, and it seems to be due to a strange metal (possibly radioactive) that mysterious men are mining for unknown reasons.