Let the hunt for the compendium begin! You would think the king of hell would have the power to take down one angel, I mean they did have the other one captured and tied down at the beginnning of the episode.
Castiel likely got a bigtime power boost when he was resurrected (where do angels go when they die, anyway?
) by God. I think he was just a basic-level angel when he first appeared in Season Four, but after everything he's been through and done since then I think he's probably one of the strongest now.
I mean, he was showing off a level of power we'd never seen before from him when he got all glowy-eyed and badass with Crowley. That suggests to me that he's not your average angel anymore.