b. Destroy/Conquer Stronghold: If the attacker won the battle against a stronghold, he removes the opponent’s stronghold token. He may then replace it with one of his own stronghold tokens (with the damaged side faceup). If all of the winning player’s strongholds are already in play, he may voluntarily destroy one of his strongholds from the game board in order to replace the enemy stronghold.
Yeah, it doesn't seem obligatory to me either, and the FAQ has nothing to say on it.