Castriel's personal guards died one by one--they're too weak to protect him. So, he hired an assassin from the assassins affiliation that you're in.

Your chief has gave you the job. But when you arrive at the palace, Castriel was disappointed. He thought he'd get a strong male assassin, and he thinks that you aren't strong enough.

He disguise you as one of his concubine, instead of announcing you as his assassin. So you won't get attacked or discovered, because he thinks you're too 'weak'.