T.I. may be detained behind prison walls right now, but he's still putting out music. His next single from upcoming album No Mercy is all about feeling trapped inside another kind of jail. On "Castle Walls," Christina Aguilera lends her pipes to the mournful hook, which bemoans the loneliness of being a prisoner in a gilded cage. T.I. brings his signature smooth rap over the beat, so it doesn't feel too soft and emo. It's a powerful track, and a strong taste of No Mercy, which drops Dec. 7. Listen to the song when you read more.