Way back when in the days of 31.18 (or something like that), I made a fort, trained a weaponsmith up to legendary with a metric shit-ton of serrated iron, silver, and copper disks and a few iron warhammers here and there, got him to make a few masterwork adamantine spears, and abandoned it so I could explore (and more importantly, plunder) it as an adventurer. Before I could get one of the spears, I ended up face to hideous face with one of the nearby cavern's forbidden beasts. Turns out the grates I installed in the water source about 20 z-levels down to keep unwanted guests out had broken down and this thing, an ankylosaurid if I recall correctly, which is like a dinosaur but worse 'cause it's got armor, had gotten in.
I still had an iron spear at this point and maybe about three companions to ward off boogeymen.
For the most part during the fight, I was the only one doing any real damage because my character had superhuman strength owing to his demigod stat points heritage and even then, I could barely do anything other than the occasional boo-boo.
Then I fractured one of its bones. And then another. And then I tore its guts. It passed out pretty quickly, but we were barely any closer to killing it. None of us, me included, could pierce its skull, so we had to settle with making the damn thing bleed out or something. And so we did. And so we came out afterwards covered in sweat, blood, and vomit. I got a bitchin' adamantine spear (which I modded to do slashy attacks afterwards) to show for it, though.
Also, apparently, if you abandon a fort while an ambush party is still hidden somewhere on the map, the ambushers are considered to have conquered the fort and will be found inside afterwards. Makes sense that they would be able to conquer an empty fort, I guess.