Mark Tidd's citadel is a children's adventure novel set during a summer vacation. A group of friends arrives at a summer hotel, only to find it abandoned and in disrepair. Faced with an unexpected situation, they decide to rent the hotel and take on the challenge of managing it. Their adventure takes an intriguing turn when they meet a boy who seems to be hiding from mysterious pursuers. As the group settles into the hotel, they begin to notice strange occurrences, building an air of suspense and mystery. The story unfolds as they attempt to protect the boy and uncover the truth behind his situation. The novel weaves together themes of youthful determination, teamwork, and the excitement of solving a mystery. The boys face dangers that test their courage and loyalty while navigating their way through a series of challenges. This tale of friendship and bravery in the face of danger provides a thrilling journey, combining humor, suspense, and mystery throughout their summer adventure.