We know it was Castiel who broke into Lucifer's cage to release Sam's body, but has it ever been explained how he done this? I was thinking that he used the Rings of the Horsemen, but didn't Death get his one back? Remember how Raphael intended to do the same, but to release Michael and Lucifer instead? He was an Archangel, obviously far more powerful and knowledgeable than Castiel, and yet he seemed unable to do this?
Moving on, even after breaking into Lucifer's cage, I don't see how Castiel could simply walk in and grab Sam's body, when two of the most powerful beings were in there too (not the two most powerful, but definitely in the top 5 beings we've seen in the show). Surely Michael and Lucifer could have escaped once it was open, as there's no way Castiel could evade an Archangel or subdue one (let alone two). The only other other being to have performed such a feat was Death, but him being able to do it is understandable.