Originally posted by indigo:

Uh, that's happened to me, several times. There is usually some message about it, just before they start spawning in in waves. It's not particularly difficult, more annoying, as this usually happens when it's getting dark, I'm exhausted from spending all day cutting down trees and picking up bits of this and that to build that other thing I need, in order to build that thing I want, to be able to build that other thing I really need.

It gets progressively harder as you progress the game. Grey dwarves might not be a problem. But multiple trolls that can smash your stuff in one hit or take away half your health in a hit is a different story.

Combine that with a few mobs that wander in because it's night time. Then it's raining and cold so you aren't stats don't regen like they usually would and it becomes an issue.