This achievement is the first part of Pursuits: Subversion.
There are two possible ways to unlock this achievement.
This unlock has been removed for Order in Patch 1.4.4
Kill Dirk Eisenhauer in Salzenmund and loot a Wizard's Key from his corpse. Dirk walks from his spawn point to a tent at around 50, 55k, a bit north from the treasure chest mark on your map. He is unattackable until he finishes talking to his friend and receives a key. If you then right-click this key, you will be teleported inside the tower. Note that he only drops one key per kill so you will have to kill him multiple times to bring your whole group into the tower. This allows you to progress to the second stage of the overall pursuit achievement, in which you slay the level 9 hero at the top of the tower.