Baldur's Gate II: Enhanced Edition

Complete the quest "Animal Trouble in Trademeet" with a Main Character whose class is Druid. The stronghold is unlocked after defeating Faldorn and accepting to take over as leader of his grove.

To get the Druid stronghold, choose Druid as the class for your main character. Go to the City Gates in Athkatla and speak with Flydian, whom is standing in the middle of the street. He'll give you the quest "Animal Attacks in Trademeet". Complete the quest by challenging Faldorn to a duel for the leadership of the grove - not by poisoning the well. Win the duel, finishing speaking with the druids in the dialogue afterwards, and the achievement will unlock.

Note: This can be done immediately after the prologue and speaking with Gaelan Bayle.