Good Old Secret Seven by Enid Blyton

When Jack brings a wonderful, heavy brass telescope to the garden shed headquarters, the Secret Seven are absolutely thrilled. They take turns scanning the countryside, but when it is Janet's turn to look, she spots something highly suspicious on the massive, rocky hill in the distance. The towering stone ruins of Torling Castle are supposed to be completely abandoned, but Janet watches a flock of black jackdaws suddenly scatter in a panic, and then clearly sees a man's face peering out from one of the crumbling tower windows! Determined to find out who is hiding in the dangerous ruins and why, Peter, Janet, Jack, Colin, George, Pam, Barbara, and Scamper the dog immediately launch an investigation. But snooping around the dark, unstable castle is incredibly perilous. The children quickly find themselves trapped in a massive, high-stakes adventure with a dangerous gang of criminals! Caught in the ruined tower with no way out, the Seven will need a daring rescue from Peter's father and Matt the shepherd to escape the castle before the villains silence them forever!