Argh... My fortress was doing very well, and of course something bad had to happen. Somehow a dragon appeared in my fortress, I have no idea that where its came from, probably it came in via the main entrance??..
Anyways, its killed most of my dwarves, including 11 from my dwarven axeman. I made a huge mistake as well..my patrol armies contained only 2-3 dwarves, I guess a 10 dwarf main patrol should guard the main entrance area.
The other mistake is that I never built gates and levers near the main gate. But hell knew that monsters can walk in like this.
I guess I will have to restart the game, since my fortress is now full with blood and dead dwarves.