Instructions
Play the game until a stronghold mission becomes available. These missions are usually accessible once you complete a number of missions in the stronghold's area.
Accept the stronghold mission and proceed to the stronghold. It should be the only thing marked on your map during the mission.
Complete the stronghold mission by killing the leader of the gang. You and your gang will have to kill many opposing gang members to get to the leader. Once you kill him, the stronghold and the territory are yours.
Return to the territory after the game informs you that it has been retaken. This will happen at a random point sometime after you take the territory.
Kill the marked men in the territory to take it back. Once you have taken the territory back, it's yours for good. You will not need to defend it again.