My first siege occured, just after a human caravan had got to my fort.
One of my axedwarves was for some reason patroling separately from the rest of his squad, and tried to take them on single-handedly, only to be slowly beaten to a pulp by about ten mace goblins. (He lasted surprisingly well, although didn't seem to actually do anything useful other than delay the gobbos while I assembled all my other soldiers to the front gate).
I then ordered my army to attack - and was completely massacred, without even killing any goblins. My military was reduced to 5 dwarves who had been too lazy to join the others on the front line.
So I locked down the fort, and started building loads of weapon and rockfall traps behind the gate.
While I was doing this, the traders finished trading and "left". Then a bit later their diplomatic liaison went berserk and had to be put down. (Fortunately, my reorganized military were able to do that at least, and no-one else was hurt).
Eventually, I finished building all my traps. I stationed my military inside my entrance tunnel (so they wouldn't blindly rush the goblins as soon as I opened the gate, but would be there to stop any that got past the traps).
Pull lever
The goblins marched towards my fort...
The tame giant jaguar that was wandering around in my above-ground walled compound charged the goblins...
The human traders rushed to escape my fortress...
Cue massive bloody brawl in the entrance to my fort, as the goblins attacked my pastured animals and the traders, the caravan guards attacked the gobblins, my traps did far less than I had expected them to, and my military were held back until there was nothing left to do but finish off wounded or fleeing goblins.
On the plus side, I was able to recover about half the trade goods I'd sold to the humans.
I can only imagine what excuses my diplomats will have to come up with to explain all that.
(Well, given that they're dwarves, probably something like "It was his own damn fault. You shouldn't send diplomats to a dwarven fortress if they're the sort of elf-minded weakling who'll go mad from being cooped up underground for a few weeks. It's not our fault we had to hack him to pieces with our axes. And if your traders are stupid enough to rush head first into a goblin siege, after we've gone to all that trouble to set up traps for them, then they deserve everything they got. And if they can't look after all the goods we made for them, then its only fair we take them back. I don't see what you complaining about. So what if you lost six men including a noble? We lost 25 braves dwarves trying to defend you, and you didn't lift a finger to help until you tried to make a run for it. Pah!")