User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-190.92.7.204-20150612220839/@comment-5961531-20150619203303

Castiel was only a regular Angel at the time due to his borrowed grace, and even Alastair defeated Castiel in season four. So, Cain defeating Castiel isn't THAT big of a feat. Also, just because Cain said he could have killed Castiel doesn't mean he actually could have. Cain didn't have the First Blade or an Angel blade, so how else could he have killed him.