Set the North, South, East, and West Sentry Towers on fire by using the Torch of Retribution inside each of the buildings.
Listen well, [player]. While the four towers can never be destroyed, you can use the torch of retribution to set them ablaze for a very long period of time: A fire that no element or force can subdue.
While they are ablaze, no guard or sentry unit can inhabit the watch posts and thus, their first line of defense is nullified.
You must enter each tower and use the Torch of Retribution within their fortifications. Beware the lookouts posted by the Dark Irons, [player].