So I had this fortress I called Coldgraves. I am a firm believer that in dwarf fortress one sets one's own challenges, so my idea for Coldgraves was kind of an eskimo-dwarf kind of thing. No farming allowed, no use of mechanisms, no metalworking. Hunting, fishing, and trading for things only. All constructions had to be above ground, of wood, and could have no more than a single basement with a footprint not greater than the building above it. The biome was near freezing, and the river thaws for only a few weeks each year.
Sometimes dwarf fortress teaches us lessons we already know. I had the fortress up to about 80 dwarves, with a communal longhall, trade depot, kitchen and butcher's shop, and so forth. Everything was going well. If you're a chronic above-ground builder like I am, you know one of the greatest challenges is maintaining your wall. A wall is completely necessary, since just locking yourselves inside buildings is not really a very good defense against sieges, but it eats labor and raw materials as only a megaproject can. So I was expanding the fort to include a barracks, but I discovered that my wall was one tile too close. Ideally, when expanding outdoor walls, you build the entire expansion without deconstructing any part of the existing wall first, so that at no time is your settlement vulnerable. Then you deconstruct the inner wall. But that's a lot of work, and it was just a few little tiles of wall. What was the worst that could happen?
*Urist McCarpenter begins deconstructing wooden wall*
An ambush! Curse them!
An ambush! Curse them!
An ambush! Curse them!
Yes, it turned out there were three squads of goblins loitering just outside the wall. Though I had a competent military, I didn't have enough. They killed about half the goblins before they were themselves killed by marksgoblins (leather armor and wooden bolts are not so hot). The goblins flooded the fortress, killing about 20 dwarves before I successfully got the rest inside the long hall, where I also keep my food for just this kind of emergency. Then I locked the doors and witnessed the worst tantrum spiral I've ever seen. 60 dwarves locked in a 2 z-level 10x10 building. Tempers flared, and the living killed the living until they were outnumbered by the dead. Wives murdered husbands, parents murdered children, and brothers murdered sisters. The most shocking moment came when a seven year old dwarf tore his own father's throat out with his teeth, before being mauled to death by a war dog. The beds contained more corpses, skeletons, and body parts than living dwarves, and the room was constantly purple from miasma. There was nowhere to put the corpses. Those few dwarves left alive slept next to the rotting corpses of their loved ones.
The dwarves lived in that hellhole for over a year. Eventually, of the 60 dwarves who survived the ambush, only 6 remained. But the people of Coldgraves are a harder sort than most. The settlement was a nightmare of corpses, trash, and buildings covered in blood, but I refused to give up. When the goblins finally left, I walled off the fortress from the outside world, including the trade depot, and vowed that the gates would not open again until that catastrophe of the great purge had been undone. My six remaining dwarves spent the next four years hauling clothing, cleaning blood, and burying the dead. Coldgraves is renowned for its expansive and picturesque above-ground graveyards. Finally, after a lot of sweat and toil, coldgraves was ready to function again. After eight years we began getting immigrants again, and reopened trade with the other races. Coldgraves' sole export for the next seven years after that was the clothes and possessions of those who had died. But we did come back from it. Coldgraves is now a sprawling settlement with many buildings and over a hundred dwarves. We have a hospital, wells fed from underground cisterns, a jail, fortress guard, and more sheep than we know what to do with. We are famed far and wide for our masterwork bone-crafts and high-quality dyed clothing. Our crack military of marksdwarves, axedwarves, and giant badgers keep all enemies at bay. We still have problems, but on the whole, life is good. I'm even considering allowing the mining of stone in order to build a great castle to look over the settlement.
In the end, Coldgraves taught me an important lesson: with time and tenacity, any fort can be saved. I send this message to all those who have posted in this thread. Don't give up.