I went there to mess around during my trial with my pally the other day. There's a door and "flight path" above the back entrance near the top of the tower. Was that used for a quest? I don't remember that exit from back in the day.
It was always there, I believe; it's viewable from inside if you explore a bit after Chess rather than going straight to Malchezzar. I believe there was talk once upon a time about a plan that never came to fruition about a third distinct entrance (After the front door and the 'back door'), which was intended to be connected up with that flight point.