Elder Scrolls Oblivion GOTY Arcane University?!?

I just recently earned the rank of Archmage in the Mages Guild and for some reason I can't gain access to this one specific area! When you walk into the Lobby, their is a door in the center, but every time I try to open it, it says its locked and requires a key! Is it possible to find this key, and if so, where?