Idea for an historical novel
A spy is immured in the walls of a castle as a punishment but discovers that he can travel around the whole building through gaps and empty volumes in the masonry. He is able to observe and come to understand the whole life of the castle and its inhabitants. There are a few places where he can reach the outside world and is able to steal food, clothes and so on, but also to start moving or leaving things, so affecting the behaviour of the inhabitants and bring about the collapse of the regime he was spying on.