Elder Scrolls IV. Cant find the key to the Citidel Hall Door?

Ive gone up 3 levels of the tower & now I'm stuck. I have to use a key to go through the Citadel Hall Door but I can't find it anywhere. I've searched all the bodies & gone through it again & again & still can't figure it out. Am I missing something or did the game glitch on me? I also tried looking up glitches but couldn't find anything about this. Where is the key?