"Mark Tidd's Citadel" by Clarence Budington Kelland is a children’s adventure novel written in the early 20th century. The story follows Mark Tidd and his friends as they venture to a seemingly abandoned summer hotel for their vacation, only to discover it has not been operational for years. Faced with the challenge of their unexpected situation, they decide to rent the hotel and take on the adventure of running it themselves, all while encountering a mystery surrounding a Japanese boy named Motu, who seems to be hiding from unknown pursuers. The opening of the novel introduces Mark Tidd, his friends Plunk, Binney, and Tallow as they arrive at the long-desired summer destination, only to find the hotel closed and dilapidated. The laughter quickly turns to concern as they realize the implications of their predicament. However, Mark's determination shines through as he swiftly proposes they reach out to the hotel's owner and negotiate for a rental. As the boys settle into their adventure, they also encounter strange happenings and an air of intrigue when they discover that Motu is connected to an impending danger, setting the stage for their coming challenges as they attempt to protect him. (This is an automatically generated summary.)