The first fort I've had where I've gotten a military started, food supplies sorted out, and the beginning of a good defensive perimeter (moat being dug, legendary dining room, enough rooms for all my dwarves)...
Then 25 migrants show up putting my population at 80. I figure, I can handle an early goblin siege without too much difficulty, or I could turtle.
A siege comes...but its undead. 80 undead battering down my door. I quickly get almost everyone inside (10 or so left outside), and raise the drawbridge....only to realize that it literally raised, and I had built it to raise up instead of in a direction. I quickly get all my troops to the main hallway to defend it, and they do well. They do so well that they liter the hallway with bits of corpses...
Then the animals I left outside get slaughtered and torn to pieces....and the undead hit my refuse pile. Now I'm being attacked by pieces of my animals, pieces of my kills...yak hair...all sorts of fun. My marksdwarves run out of bolts, and suicide themselves. My melee dwarves hack everything ot bits, and while I'm now down to about 50 dwarves, the undead are down to about 40, even with their reinforcements. Then my dead dwarves, and their corresponding pieces, come back to life, as do a large number of wild animals the undead found time to kill. Enemy count goes up to 175, and no matter how many dwarves I throw into the breach, we are overwhelmed. The undead breach our halls, slaughter everyone, except one child...
The child spend the next 30 minutes being chased by his friends and family across the whole map before finally being cornered and ripped to pieces....by his own mother (wish I had a screecap of that).
How the frick are you supposed to defend against this? I saw no necromancer, I'm not in an evil biome...wtf?