Actually you don't need to fight any guardian. Just wander back towards the entrance, and in a doorway between the series of rooms (and pairs of ghouls and ghosts) between the first 'roof' and second 'roof' areas you will encounter the dialog box about the keys.
For anyone who read the answer but still couldn't find the key (like me ) it is hanging on the wall by the door that you just entered (coming back from where you got the quest).
It is in the long room between the two prison towers.
) it is hanging on the wall by the door that you just entered (coming back from where you got the quest).