I think I screwed up a burrow. I am building an actual castle on top of the fortress, including extensive fortifications for my marksdwarf squadron. There is a raised entrance with a drawbridge, a couple of towers to protect it, and a keep. The keep is the home of the Captain of the Guard, and includes a barracks and eating hall. Ammo storage is in the basement.
Goblin attacks have killed off two elf caravans, and driven off the last dwarf caravan. Annoying, really--I need to activate the military as soon as a caravan turns up.
I recently had a siege. I set up a burrow, and brought all of my civilians inside. I activated the military, with one squad of marksdwarves in each of the front towers. It was a large-ish siege, with about thirty or forty trolls and goblins. I peppered the approaching goblins squads, and placed my military near the trade depot inside the walls of the castle. I gave the order to pull the lever to raise the drawbridge. Then I realized that the melee squads were rushing the goblins. Fortunately, the drawbridge did not raise. Unfortunately, that is because I accidentally opened the secondary entrance.
At this point, my dwarves swarmed out to collect the goblinite booty. Panicking that I was about to lose a large portion of my population, I unleashed the melee fighters on the goblins, and re-positioned the marksdwarves to take potshots. Fortunately, the goblins were quickly driven off, with maybe five goblins escaping.
End result--about thirty trolls and goblins killed, with the loss of one marksdwarf and seven civillians.