From the rules, page 8:
Purchase Strongholds: A player spends gold to purchase strongholds from his Barracks.One could argue that this doesn't cover how secured (battled for) Strongholds enter play, but the rules also say (page 7):
Purchased strongholds are placed upright (refreshed) above the player’s Home Realm.
1. Refresh Exhausted Cities, Strongholds, and Rewards: Where "original position" must be how the cards entered the player's area in the first place.
During this step, the player refreshes (rotates to original
orientation) all cities, strongholds, and Rewards under his
control. They are now ready to generate more influence.