Recently did this for the first time, and after the patch. Need to kill 20 guards, loot 5 keys and free 5 prisoners. After we killed 20 guards, we had looted only 1 key /sadface.
Nearly 20 guards later, and countless other mobs, finally get 5th key. Also, 4 of our keys dropped on guards high up in the keep. Just a heads up for those who haven't tried it post patch.