The achievement is unlocked when you complete a series of quests to restore Gravehal Keep as part of The Legend of Dead Kel questline. It can be done before the main story quests are completed or afterward in post-game play.
Talk to Padrig (Paddy) Dower just outside Cape Solace to the north and accept his quest, Gravehal Keep. Concluding it triggers a series of quests. The quests require five types of construction materials. In total, you’ll need: Beachstone (12), Bearhide (9), Emberwood (14), Sailcloth (11), & Seasteel (13). Note that if you pick up any of these items excess to the requirements, you will use up an inventory slot for each different type of item. They are considered quest items and cannot be junked, sold, or placed in a stash chest. To completely restore the keep you’ll need to complete the following quests:
The achievement unlocks when you talk to Paddy to complete the Gravehal Tower quest and conclude the Gravehal restoration series of quests.