It was directly after my largest Overworld creation: Three Towers that reached the full height of the map. They held all of the actual production work.
I got a Siege; Nightwings. Fortunately, at this time, I had been creating tons and tons of steel armaments in the event that this would happen. HOPING this would happen.
But it was better then I expected. Those dwarves were out of their league. I sent them out, 80 strong, the entire Weight of the Fortress bearing down on my foes(The entire Fortress was militarized.), but they slaughtered them mercilessly. Valiantly as they tried, they could not overcome the fell waves of Nightwings, and finally, there were but five dwarves left. I had locked them in the Central Tower. Quickly thinking, I locked each of the Towers, confident that they could use the roof as a pathway..Which, I'll give it to them, they did. The Nightwings were really too stupid to path inside from the top, so I had plenty of time. After making sure that the crops were stored away, I had my one and only still able to walk dwarf start tending to the wounds of his comrades. Only one was saved. Only one managed to make it out without starving or dying of thirst. Finally, I waited until I could wait no longer; one of my Dwarves had wandered outside. He was going to die, if I didn't take action. So I, impatient with the battle, finished it; it was 2 Nightwings, and 2 Dwarves. And the Dwarves won.
Out of memory of the greatest battle ever known to me, I was going to leave the bodies out there, but then another Nightwing Siege finished them off.