User blog comment:Chaos11394/Where Do the Demons Go?/@comment-96.225.135.231-20160101232800/@comment-27464255-20160102102114

That's exactly what I was thinking the moment they said that. I don't think Castiel ever got there, but even if he had, I don't think it's impossible for God to bring him back. It's God we're talking about. Besides, it's possible either Sam or Dean end up there, and if they do, they'll eventually come back, so it's possible.