So, I just curbed an 80+ zombie siege in my fort with a giant bridge deathtrap hallway... I made a hallway 40+ tiles long, put four bridges in it, connected them all to a single lever.
Then, I put a stairway at the very end of this hallway, got just up to the surface, then I burrowed four miners right by the thin floor still seperating the hallway from said surface. I told them to mine out downward steps, unburrowed them, reburrowed them far inside the fort, then set the lever to repeat. I helped the miners make their escape by building a bunch of fortifications behind the upward stairs at the end of the hallway, then plopped my marksdwarf squad behind it. They cut down the initial wave of zombies in time for my miners to make it to safety. The only dwarf I lost was a random dwarf baby who got clawed by a giant bat 50 levels below.
One thing: I made one tiny hole, and if I didnt have a trapped up hallway Wed be into Markedangels: Reclaimed. I.e., Ive already tried something very similar, but with Cage Traps. Lots of them. They swarmed the hallway in seconds.
Its a good idea, though. It could be adapted to work. Just consider that almost everywere on the surface has a horde, and some of them are moving. And I think We have more than 80 problems up there. Burrows were ineffective for some reason.
Additionally, Were all a bunch of depressed, some of us borderline enraged, survivalists. Many of us are injured or maimed already. And We only have x1 Pick.
Itd need to be tweaked, quite a bit, but again, not a bad idea.
Im thinking that Wed need two Hallways, a Suicide Dwarf, and another Pickaxe, before We even considered this.
We lost all our other Pickaxes the first time We dared open one tiny 1 pixel hole, in an empty bit of land free of zombies. The Door was sealed from a smoke cloud rougly 2 minutes later, after everyone blundered outside, despite most of us being burrowed. Most of us proceeded to die, either from Zombies or Berserkers.
Besides - Bathing the World in Magma is vastly more awesome, and suitable, since someones already proposed it ages ago
And theres Goblins.
EDIT: Optimally, just read this, for effect:
Armok save me, what the hell is this. I've followed your journals, but seeing this mess myself now...I really fear pressing SPACE now... ;___;
Hoo~ boy...