To get the Warrior stronghold, choose Fighter as the class for your main character. Go to the Slums district in Athkatla and speak with Nalia de'Arnise inside the Copper Coronet. She'll give you the quest "The de'Arnise Keep has been Invaded". Travel to the de'Arnise Keep, complete the quest, making sure Nalia stays alive, and she'll give you the offer to become the new de'Arnise ruler. Accept, and the achievement will unlock.
Note 1: This can be done immediately after the prologue and speaking with Gaelan Bayle.
Note 2: Loot everything inside the keep before finishing the quest, because the keep will be "replaced" with the "stronghold version" of the keep once you accept Nalia's offer, and everything inside will disappear.