Tell me about it, my latest fortress was doing great despite me not bothering with propper management. It even defeated the first siege with relative ease.
Then the second siege came along.
To many goblins to count, many of them were riding cave crocodiles and freaking jabberers. Not to mention the blizzard men that came to rape the civilians in the arse whilst the military tried to fight the overwhelming goblin force.
I reclaimed and I'm still churning out coffins for the fallen. Upon inspecting the mess left behind and the now friendly goblins and beasties I found some ghastly war trophies, one I remember seeing was a dwarf nail bracelet owned by a crocodile.
At least it gave the fortress some character, it needed some given the serene embark. First artifacts to be made was a coffin, followed by some items depicting the brutal slaughter that was.
I had a similar (thankfully more survivable though) incident with my second siege in this fort. Mind you, I've only ever endured 3 in my entire playing experience, so I'm still new to it. The first siege I received in this fort (and the only in my previous and abandoned fort) were fairly small and repelled easily. I lost no dwarves on one occasion and lost a single one to a broken spine in the other.
The second siege I received in this fort though was a wake up call. I'd been intentionally letting things run freely to spice it up a bit (such as breaching the caverns and twiddling my thumbs waiting for something Fun to happen), but I didn't expect the goblins to be so nasty suddenly. I'd heard that goblin archers could be monsters, and a squad of them showed up riding camels and jaguars. I lost about half of my militia to that fight, either immediately in combat or due to infection / bleeding out in the hospital.
Lesson learned: finish the wall, carve out some fortifications, get more marksdwarves and make sure they're all fully armored. So many broken legs because no one had any leg armor...
Oh, and make sure your doctor isn't busy hauling stone around when he should be y'know, sewing up Urist McBloodFountainTheGushing. And have soap.