Dishonored Guide: The Empress’ Secret Room in Dunwall Tower (Finding the Bone Charm inside the Dunwall Tower)
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.
There is a video with a visual version of this guide below.
Dunwall Tower main mission: Opening the Regent’s Safe in the Dunwall Tower and Eliminating The Lord Regent Non-Lethally
My interest was captured when I used the heart and saw a Bone Charm behind what seemed to be a solid wall: