Note that Tactics Cards can be played by both players as appropriate during this battle. There are a couple of such cards that the defender can use to give the attacker a little surprise, if he doesn't bring enough units to compensate.
- Summon Lightning, which would deal 4 damage to the attacker. (Also the defender, but if the defender has no units, there's no risk there). It can be especially nasty if you play two, dealing EIGHT damage!
- Scorched Earth. Doesn't really affect whether the attacker wins, but destroys the Stronghold so the attacker can't use it.
I'm sure there are a couple others that I'm not thinking of.
Note that attacking an "empty" stronghold DOES still count as a battle, so if you are using Mobilize, the other activation cannot be a battle.