Well, my first necro ambush has occured.
A few goblin snatchers preceded it, and were summarily executed by my small but legendary militia. 2 axelords, 2 spearmasters, 2 skilled/expert macedwarves, and 2 marksdwarves are all the fort needs... for now.
Anyways, the first necro was detected soon after the 3rd goblin was put down in a spray of ²'s (thank you, axelords) and as I gave the order to dump the bits into the volcano, they reanimated! A passing dwarf saw the first necromancer giggling and gesturing outside the gates, and the militia bum-rushed him, neatly bisecting him. These parts were also tossed into the volcano, but not before the hidden necromancer (or several) thought it would be funny to keep resurrecting goblin and human arms and hands on the gate, right next to the milita.
No wounds were suffered by the fort, and all the bits have been dumped. The necromancer(s) remaining in hiding have apparently thought better of invading, and left for now.
Oh, and my fort now has a great self-destruct "button". Just need an engraver or miner to punch though a wall, and the whole fort will flood with the blood of the earth.
Yep. The volcano will drain right into the main fortress proper, and keep filling it until there is nothing left but magma.
And as soon as I post this, and unpause the game, a necro decides to jump into my cage traps. Excellent. His knowledge of the secrets of life and death and immortality will be of much more use here than in his tower.