In the final mission of the game, after entering the Dunwall Tower Zone (second mission area), work your way straight forward past the wall (I went over the roof), then keep going along the wall line on your left, rather than heading straight toward the mission marker and into the Tower. The Gazebo is kind-of past the tower entrance, overlooking the water (it's hard to miss, past a large bonfire). Once inside the gazebo, interact with the plaque to unlock the achievement.