User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-4011543-20111223220120/@comment-76.212.203.73-20120226071859

The way I interpreted Castiel wanting to keep Michael in Lucifer's Cage, is he was concerned the other angels would defect to the archangel's side in the event of his return. As such, Castiel would not be able to stand alone against the combined forces of Michael and the Heavenly Host., regardess whether he was indeed more powerful than the former or not.