I had my first siege last night, a squad of goblin axemen led by a lasher and, for some reason, an Elk Bird.
They charged into my front entrance (the ones behind that were closed by drawbridges) and killed the bait animal that was there. I pulled the front entrance bridge up behind them, leaving them trapped on a small island, practically on a shooting range for my squad of marksdwarfs.
Now, of course, the marksdwarfs weren't doing anything. They had quivers, they had bolts in them, they had crossbows (only one per dwarf!), and the same squad had previously killed something by shooting at it. I guess they just thought they were too far away or just didn't care.
I closed the front bridge, and the goblins nicely stood all on it, so I pulled it up again. They landed in a hole under it, which I then proceeded to fill with water until it was swapping 6 and 7, which was when they started to try and escape (resulting in me closing the bridge again on them).
The trapped goblins ended up going between drowning and not, getting Winded in the process and I suspect eventually they'll suffocate.
The ones who escaped weren't that lucky. Two of them were caught by my squad of 8 legendary axedwarfs, and those do horrible things to their victims targets. The last one escaped to the outside, triggering a trap and getting his arm broken by it, until he was just laying on the ground in "Extreme pain", according to his wounds display. So I sent my squad of swordsdwarf trainees with wooden practice swords after him so they can skill up for a few months.