Its levels are accessed via a central spiral staircase. The third level is not accessible until you defeat The Guardian: the staircase meets a dead end before reaching the third level door, this wall disappears once you have access.
Dungeon (1st and 3rd Levels)
Once the room is cleared activate the four Resonating Crystals. Two of these activate by being hit by an Aard (gaining a green aura) and the other two activate by being hit by an Igni (gaining a red aura). Hitting a crystal with the wrong sign resets all crystals.
Once all crystals have been activated the door to the next room opens.
Picking up the sword causes a Vran Guard (simply labelled "Guard") to appear and attack you. It looks like and fights in much the same way as The Operator except that it summons groups of wraiths instead of two gargoyles when invincible. It also does not summon wraiths every time it becomes invincible giving you more opportunities to attack it.
Once it's defeated, you can move on to the next room.
You can exit this room revelling the true bottom of the spiral staircase which has piles of loot stacked at the bottom.
The Secrets of Loc Muinne (2nd Level)
Once you accept the mission from the two scholars in the market square you meet them in the second level of the tower. There Cynthia, Adalbert, and the two scolars have dug a tunnel through the crypts to the sewers underneath the Redanian encampment to access Dearhenna's laboratory.