The Crossbowmen tried to "club" them with their weapons instead of shooting them?...
Dumb question, but did you make quivers and have bolts for them?.
Sieges are beatable, you just have to prepare with stratagey. I'll site my 40D Fortress Nitigrigoth, which gets NASTY sieges, and are getting nastier and nastier each time.
I have limited access to my fortress which is underground, the two actual entrances have lever controlled floodgates. Its all surrounded by battlements which can only be accessed from the underground fortress and allow my Marksdwarfs to run around ontop and shoot down at the goblins through fortifications. And there are close to a thousand cage traps on the surface. Designed in such a way as to break the enemy siege numbers.
Inside the fort, I have my 36 Champion Wrestlers, and at present, training Spear Dwarves. And 50+ Wardogs, all positioned at the staircases leading from the surfaces entrances. So, should a Siege be able to break through all that. They are my last line of defense.
Oh, and those levers are inside my fortress so that if I order my dwarves inside they can still operate them.
So far, I've only had one incident where an enemy siege got into my fortress. And that was through a weak spot I had not noticed. But they were taken care of. With only one champion injured. And she gained 4 titles in the process.
Even though 2010 is harder, I'm certain the same elements will apply.