Use the Cage Key to free Katya on the cage that’s hanging on the ceiling of the tower.
How to Get the Cage Key
The Cage Key is on Bram’s chamber, this room is just beside the Laboratory. Inside walk to the far right, where you’ll see a cane sticking out of the umbrella stand next to the gate. Note that that stick will not appear if you don’t have the anti-vampirism with you. Click on it to pick it up and then use it to grab the Cage Key from inside.