Wow...
What the heck are Nightwing made out of? O_o I had a medium age fortress, that had managed to rebuff about a dozen sieges(light stuff, tigers, frogs, raptors, blendec, etc). About 90 dwarves. 5 squads, 3 marks, 2 melee mainly wielding silver hammers and steel axes. All 50 decked out in full steel. Marks had a total of 5-6 elites, no elite melee but definitely some skill. I had set things up to have a long killing field I could rain bolts down on, and have upped the power of crossbows to partially counter the nerf that causes 3000 bolts to bounce off steel and only cause minimal bruising, because that feels unrealistic and annoying.
Anyway, just had 2 sieges arrive simultaneously, one of warwolves and one of Nightwing. They fought it out on a corner of my surface map, while I rained bolts on them. One Nightwing seems to have glitch-dodged through or over my wall, and ended up among my troops, and started tearing into my archers before I could do much about it. But I sent my 2 squads of full steel melee to face it. And.... 1 Nightwing took out my entire fortress. By the end, every location was in the red, it had no weapons or armor, but it was still killing slapped together squads of untrained rookies with ease.
I lost all but 2 dwarves, one of which was a child, before the single invader made his escape. This whole experience was baffling and terrifying, and leaves me utterly baffled as to how one could ever face such enemies in numbers without resorting to broken mechanics(bridges and the like) or cave-in traps...