That is something that Castiel never thought that he would have ever heard Dean say. "You work for Lucifer? Dean. Are you sure that's a good idea?" He may not be the same Lucifer that Dean had killed but it probably didn't mean that he didn't come with a different set of threats. Or maybe the same ones.
He took the beer that Dean offered him. He was not sure why Dean was so calm about this situation. "Nobody has found a way to leave this place?"