<p>To open this door, you first have to reach it from the other side.</p><p>To reach the door from the other side, go to the main hall with the big painting on the wall. There you'll wall above the fireplace. If the fireplace is on fire, just walk in front of it and the fire will disappear. Then you'll be able to break one part of the wall (shown below) using a melee weapon and open a passage to the corridor behind it. After you open the door from the inside for the first time you'll have free access to it from both sides. </p><p><img alt="" src="https://i.imgur.com/g3dkgZO.png" style="height:414px; width:631px"></p>