Had a bit of a disaster. Got a goblin siege of ~20 trolls, ~160 Beak dogs and ~160 goblins. My dwarves had recently dealt with a slightly smaller siege, so I let them deal with this one the same way. I buttoned up except for the trap corridor, which starts with 5 cages (for collecting training partners), then 8 traps, each with 1 green glass disc or green glass spiked ball (I limit myself to one glass weapon per trap to keep them from being too OP). Then the corridor opens into the main passage to my depot (which is otherwise blocked by a raised bridge). There is a pasture with half a dozen war dogs on the left, and to the right is a trench that's 2 tiles wide by 2 levels deep. On the far side of the trench is a wall of fortifications, behind which are my marksdwarves. Then they have to go north a bit while under crossbow fire to get to my depot.
The melee dwarves are waiting north of the depot.
Here's a top view (goblins enter from bottom left):
#######.....melee.#
..................#
#######...#######.#
######0 0######.#
###### ######.#
###### 0 ######.#
###### ######.#
######0 0######.#
#######...#######.#
#######...#######.#
#######...#######.#
#######....... ╬a#
#######....... ╬r#
#######....... ╬c#
###########... ╬h#
###########... ╬e#
###########... ╬r#
###########... ╬s#
.╥............ ╬.#
.║.dogs....... ╬.#
.╨............ ╬.#
############^######
###########^#######
############^######
############^######
############^######
############^######
############^######
############^######
^^^^^ ######
###################
Like I said, this dealt with the last invasion just fine, only losing 2 melee troops. And it started out well this time, lots of dead trolls in the traps, lots of dead dogs when the surviving trolls got through the traps, lots of punctured trolls and goblins.
Then a marksdwarf who showed up late came in via that corridor NW of the depot and spots a troll that was still south of the depot. Being a good marksdwarf, he fired one bolt (which the troll dodged), and then rushed forward into combat swinging his crossbow. So at this point I sent all the melee dwarves in (it was almost the time they should have gone, anyways). They proceeded to play "will it blend?" with the goblins and trolls along the way down into the trap corridor and to about the corner.
At this point the backlog of goblins and beak dogs were catching up, and I looked and saw 5 or 6 dwarves on one tile, and at least 50 goblins and beak dogs on the next tile. Almost all the trolls were dead or captured, about a quarter of the beak dogs, and about 3/4 of the goblins were also hors de combat. But also, most of the dwarves were tired and approaching exhaustion. This was when I started getting the "Urist McSoldier has been found dead" notifications. Some died because a goblin lasher got a lucky shot and broke a dwarf's bone, causing him to pass out. Then half a dozen beak dogs would grab hold and start shaking him, which is going to be fatal no matter how good the armor is. Some died because they dropped from exhaustion and the beak dogs were able to do their thing.
Since the marksdwarves had no targets anymore, I ordered them to move up through the depot and join the fray, but that took a while.
In a remarkably short time, all my melee dwarves were dead. Only a handful of goblins were still alive, but dozens of beak dogs were still going strong.
I quickly drafted a bunch of civilians and told them to move to my armor/weapon storage area, but instead they kept jumping into the trench between the wagon route and the archers. Apparently some primo piece of armor or weapon had ended up down there and it was getting assigned to the recruits, who then rushed to "pickup equipment" and found themselves in a trench with the remaining troll and a couple of goblins who had fallen down there. These unarmed, unarmored and untrained recruits didn't last long.
Once I realized what was going on, I forbade *everything* on the battlefield (d-b-f) and finally managed to get some recruits to pick up gear before heading off to battle. They were barely enough to finish off the last goblins and break the siege. Just barely.
So now I have all of my skilled military dead, but most of my civilians are still alive. A few unskilled haulers who got drafted died, but not many. All of my skilled labors still have skilled people doing them. But I have no real military at all.
Time to redesign my defenses to rely on trap systems rather than skilled melee troops. By trap systems, I mean magma flooding, dodge-fall, or other large-scale traps, not just lots of weapon traps - they're too effective to be fun.
This fort was starting to get boring. It just got interesting again.