More information: http://wp.me/p1SfP0-Tz
There are three Runes or Bone Charms (don't remember exactly which) inside the Dunwall Tower. One is inside the Lord Regent's Bedchamber, the other in what I think is the Empress' Bedchamber and one in a secret room near the latter. This guide explains how to get into it.
My interest was captured when I used the heart and saw a Bone Charm behind what seemed to be a solid wall.
When you get closer you stand in front of a fire place. Look up and to the right and you will see a light.
Which you can turn by pressing the action key. The wall at the back of the fire place will lift. The Bone Charm is on top of the cupboard inside the secret room.