Bird Cage? Well... if you look closer at the poster, you would see at the top corners, there is a faint outline of a cage, blending with the sky, but still visible.
I get the impression that the entire country is surrounded by a big cage, though...
Hey, if they can create a world where mini- cities are built underwater, they can build a city under a big cage, too.
Maybe in that country, each member of the royal family would have a bird as his/ her companion to send messages or to spy others with...