You're looking for the Dungeon area, the key to which you should've gotten from the Lich. In the Central Pillar area, head north (I think...). You'll encounter a room with the Stone Faces on either side of the door.
The door to the Dungeon area is near the well that you come out of from Adrianna's area, or the brown halls after the giant. Jump off the raised area and enter the locked gate with the key.
Killing her should be the absolute final thing you do in the game, other than getting your reward from Adrianna. After the Lich is gone and you have his staff (which is the only reason to do the castle), go kill Madda and get the Scroll of Banishing from a chest in her room; give that to Adrianna, who'll give you some presents and maybe Vampirise you if you happen to submit to the Succubi.
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