[Agema]

Bear in mind for every unit you have tied up in a fort, that's one less unit you have able to fight on the map. So you can throw more troops in and hold your opponent up (then retreat), but you're really just using troops that can fight elsewhere for holding your opponent's army up longer.

If you retreat, you take no losses, but neither does your opponent, so yes you get your army out, but his comes out virtually unscathed too.

There are situations where one tactic may be stronger than another, but for the most part it's a case of trading off some advantages over others.