14 years. On a volcanic frozen mountain. Had the addition of the snakemen race and werewolf race. My first fort that pretty well focused exclusively on military defense. A squad of hammerdorfs, a squad of speardorfs, and 4 squads of marksdorfs. I built shooting towers all over the map and my marksdorfs were brutally effective. The melee squads only ever got to bat clean up. I also built the fort in the first layer of the caverns, since that was were the water was. Walled off a good section, made it nice and safe.
Until a firebreathing flying slug FB showed up. I still don't know how he got into the fort proper. There shouldn't have been any way to get from the unsafe portion of the caverns to the part I controlled. The entire army is 50 zlvls away, outside the fort, playing with the siegers. The slug busts into the dining room and sets dorfs ablaze! Everything is burning. I draft a militia to buy some time for the army to arrive. The militia is slaughtered. The speardorfs arrive! Yay! They slay the beast, but most of them die due to burns.
Over 40 dead. The rest, tantruming. The military working hard on putting down berserk dorfs. Levers are being thrown. The gates are open. The levers that refill the forts cisterns are open. The dining room and everything below it is being flooded...which at least put out the fires. Invaders are now within the fort. Slaughter continues. The fortress falls.