Given that this was written after the manga series NANA began, I'm not saying this is written for it (though she did perform a lot for it), but it could fit very well with a situation. The relationship of two of it's characters - Layla and Takumi - the song is precisely about, in that context.
He spent most of their lives building this 'castle', to protect her from the real world, and whenever she tried to build relationships outside of them he'd disapprove. Whether out of jealousy or concern, maybe both, she was effectively locked into a cage.
The sparrow part may even talk about herself: "I kept my singing from you, which is the thing you cherish more than anything else." She knows she has to return, to recordings and things, because they're all part of the same band, but her broken heart keeps her. If she stays in the castle, or the band, she'll be near him, as a member and vocalist, but she'll never get what she truly wants: to be WITH him. Their castle prevents it. All she can do is sit and reflect on what they've gained - the fame and money - and his love, what she'll never obtain, alone in their castle.
Given that this was written after the manga series NANA began, I'm not saying this is written for it (though she did perform a lot for it), but it could fit very well with a situation. The relationship of two of it's characters - Layla and Takumi - the song is precisely about, in that context.
He spent most of their lives building this 'castle', to protect her from the real world, and whenever she tried to build relationships outside of them he'd disapprove. Whether out of jealousy or concern, maybe both, she was effectively locked into a cage.
The sparrow part may even talk about herself: "I kept my singing from you, which is the thing you cherish more than anything else." She knows she has to return, to recordings and things, because they're all part of the same band, but her broken heart keeps her. If she stays in the castle, or the band, she'll be near him, as a member and vocalist, but she'll never get what she truly wants: to be WITH him. Their castle prevents it. All she can do is sit and reflect on what they've gained - the fame and money - and his love, what she'll never obtain, alone in their castle.