I don't have the save anymore, but it occurred to me to ask this so I understand maybe what happened.
I had a sizable fortress, decided to build some concentric walls outside with a single line of gates in a row--lined up conveniently for use with a ballista. Anyway, only the outermost gate had a door, for locking purposes.
Well, surprise, a giantess comes angrily up to my fortress and... stands outside the door.
Is she beating it down? I set my dwarves on alert, and wait... and wait... *quiet bored whistling* Doot de doo... is she done yet? I dismiss my dwarves, they're getting hungry.
Continue about my daily activities. Construct internal farmlands, tunnel out chambers, finish building the walls (the door isn't even locked!), build a siege workshop, build a ballista, outfit all of my dwarves with the latest in bone crossbow technology, and still she's just kind of... sitting outside my gate, waiting for a dwarf to walk out into her mouth I guess? I certainly can't attack her since she's at a choke point so my dwarves would have to go one at a time.
Several seasons go by, finally I get the message "A vile force of darkness has arrived!", dozens of goblins. They run up, and now I start getting scared because if she busts down my doors and traps, the goblins will be able to...
Huh. The goblins killed her, and she killed most of them, the siege has been broken and they run.
* * * *
Anyway, why was the giantess just sitting outside of my fortress door? I even tried opening it by letting my pets pass through it, she didn't move an inch except to chase the animal once it had left the fortress, kill it, then run back to her sitting position.
I don't _MIND_ that she was just chilling out, but it sure would have been more interesting if she had rampaged through even a single door or something...