There are about 200 different ways to possibly release Death from imprisonment. Regardless of the method, the very act of releasing him will break a seal - it is Death's release that is the seal itself, not necessarily the method of his release (though some of those are seals in themselves). Unfortunately, given that the Death of old has retired and Dean Winchester has taken his place, releasing him is not even close to a question. So Castiel will just have to be careful.

There is a reaper standing by, watching him skeptically, but there is little she can do other than watch and wait for her new master to be set free. She has been significantly weakened by her own imprisonment, and though Castiel had offered to restore her strength she had adamantly declined his assistance. She does not seem particularly inclined to trust him in regards to her own well being, though she does seem to trust him with the task of getting Dean out of the mystical prison he's spent the last quarter of a century in.

Things are needless to say, a huge goddamn mess.

Castiel speaks the last few words of the incantation and finishes the spell with the sacrifice of newborn calf. He feels the bonds keeping Dean trapped snap, and then pulls open the magically sealed casket and looks down at the figure laying within.

"Hello, Dean."