I like to think when the Army Arc is fully implemented Goblins might actually be abe to attack in numbers and combinatioins powerful enough to overcome a fortress with no military just by sheer number of bodies thrown at your traps. But then, I also like to think that my dwarves have brain cells.
Whoah, this entire thread has been confusing the hell out of me. I haven't had a long-lived fortress since 38c, so maybe this is a recent development, but the phenomenon described above is *exactly* what usually happens to my fortresses.
Generally speaking, the first ten or so sieges go very poorly for the goblins, with your typical 3x8 (or whatever) entryway bottleneck full of traps working well enough for any gobs that get past the marksdwarves... hell, unless they've brought some of their bigger buddies along, simply locking the front doors is usually enough to stop them. But as the fortress pop/wealth increases, the sieging forces get pretty huge, to the point where drawbridges become very necessary. After a few of these... well, I've only had *two* fortresses I've hung onto long enough, but I eventually have to seal myself off from the world, and incoming traders/immigrants get slaughtered by the patiently sieging mob. (though I've had some noble/hunter entourages and especially tough human traders punch through, which is pretty cool.. "thanks for saving us, guys! Now get to work! Yes sire, we have a room ready for you, don't mind the caged wolves, they're part of the, uhh, decorative ambiance.")
Once the fortress gets sealed off, things go well for a while, but moods and other occasional tragedies create a slow downward spiral into decay and madness... it happens way more slowly than it seemed to back when DF was 2D, but it still happens.
One workaround I'm aware of would be to build a drawbridge based trap (were calling those "atom smashers" now, right?) which would probably work, but that's just a little too cheaty for me. Maybe castle walls with battlements would work too. That's my plan for the fortress I've just started, and it should be a *fair* solution to the problem, providing that baddies can't smash walls yet.
So to pile a question onto the original OP's, I gotta ask: has sieging been nerfed in the latest set of releases? Or have I just done a great job of picking sites near extra vigorous goblin civs? Because like 20 or so sieges in, the armies seemed to get terrifically huge.