Actually, this could be an interesting deterrent for people who choose to retreat from battle. Certain percentage of chance for capture if retreating. Then lock the player down for a set amount of time where he can't attack of be attacked until somebody else comes over to his location and pays a set amount which will go to the player that captured him. That amount would probly be best set by a formula of how many troops and what equipment he had when he chose to retreat. So retreating with a large heavily armed army could be a costly or time consuming choice