You capture the enemy stronghold, replacing it with one of your own. The new stronghold starts out damaged and can be repaired with one of the order cards.
what page is this on? I just can't find it...:-)
pg. 21 under Battles:
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.
And then on pg. 36 it describes the order cards, the one in question is Fortify (8):
Repair a Stronghold
The player reduces his ore resource dial by one to flip one of his
damaged Strongholds so that its undamaged side is faceup. A
player may not repair a stronghold if his ore dial is at “0.”