Baldur's Gate II: Enhanced Edition

Complete the quest "Investigate the Deaths in the Umar Hills" with a Main Character whose class is Ranger. The stronghold is unlocked by accepting it from Minister Lloyd at the end of the quest.

To get the Ranger stronghold, choose Ranger as the class for your main character. Go to the Government district in Athkatla and speak with Delan, a child whom is located roughly in the middle of the map, in the center of the four squares of grass outside the Council of Six Building. He'll give you the quest "Investigate the deaths in the Umar Hills". Travel to Umar Hills, speak with Minister Lloyd(the mayor) inside his house, go to the ranger's cabin and find the journal, and then travel to the Temple Ruins. Accept. Complete the quest by finally destroying the Shade Lord, then travel back to Umar Hills and speak with Minister Lloyd again. Accept his offer to take over the ranger's cabin, and the achievement will unlock.

Note 1: This can be done immediately after the prologue and speaking with Gaelan Bayle.

Note 2: If you're making the ranger character solely to get this achievement, it can be a good idea to choose Shadows as their Favoured Enemy.