As I read the book I often thought about its title. At first I thought of it as a physical cage, but as I continued reading I realised that the cage is not in fact physical, it’s more an imaginary cage the king created to keep Mare by his side.
A big part of King’s Cage is set in King Maven’s castle. Mare has been forced to live in the castle against her will. The king captivated Mare when she and the other rebels tried to fight the king and his guards. She had lived in the castle before and the king had her by his side at all time. When Mare escaped the castle the king did not know where she was nor what she did. The only solution to his problem was to capture Mare and keep her in the castle at all costs.
In the beginning of the book I thought of the king’s cage as the place Mare lived in. She was captured in the castle, living in a small room with nothing but a bed and a couple of books. In some way this room could be seen as a cage, but it isn’t really a cage. As I read further I understood that the cage is in fact the king and his moves to keep Mare in his palace. He controls everything Mare does; she can't use her powers nor walk around in the castle as she would like. She is not free at all in the castle. She has to do everything Maven tells her to do, otherwise someone she knows will die a painful death.