It's a clever move to name this book after a popular school playground game, which will appeal not only to its intended younger readership but also their (grand)parents and ties in with the story that unfolds. The front cover is evocative too, and owes a lot to the Famous Five books of Enid Blyton. It makes you think of children racing around to find clues and help people during the Second World War, and (along with the concise chapters) conveys the fact-paced events on the pages inside.
There's also some good character development in relation to Robyn, who retains an independent streak throughout the book but becomes less naive as the gravity of the war situation, although making adult characters like the Heron so unlikable means that it's hard to have sympathy even when we are invited to do so towards the end when it becomes possible that the Heron may not be the spy in the Bletchley Park camp after all...
There's also some good character development in relation to Robyn, who retains an independent streak throughout the book but becomes less naive as the gravity of the war situation, although making adult characters like the Heron so unlikable means that it's hard to have sympathy even when we are invited to do so towards the end when it becomes possible that the Heron may not be the spy in the Bletchley Park camp after all...