The dust has been piled up by the side of the fortification, likely blown there by a light subterranean breeze after lying about loose for a while. There's a fair bit of it and now everyone knows to look for it you notice the grey mortar is missing from between the flagstones in a roughly by seven foot wide square right in front of the door, all except the ones right up against the wall of the tower which still have the mortar fixing them to the wall there.<br/><br/><div class="spoiler_container"><div class="spoiler_header" onclick="hideSecond (this);" role="button">Updated Map</div><div class="spoiler_body">(https://i.imgur.com/iW6aoLU.png)</div></div>