^ You might want to evaluate animals for use first - as far as i know, cats (of any age) are too light to trigger pressure plates. EDIT: and you can't make citizen-triggered single-use pressure plates: your mechanics will trigger and thus destroy them while trying to link them up.
For the first time ever, i received an undead siege - 43 undead and three necromancers. On my side were six weaponmasters, a random marksdwarf, 48 civilians and two sturdy doors. No traps, no moat, no wall, no bridge airlock. All workshops and farms on the completely unprotected outside.
First order of business was getting everyone into the panic room (communal dining room and individualised dormitory), via the already-established civilian burrows. The room is only accessible through a pair of doors, which were locked once all civilians were inside. Soldiers were stationed outside, to see how badly they'd be massacred and to hopefully intercept the necromancers.
Surprisingly, once the necromancers caught sight of the military, they ran away and left the site. The soldiers put up a somewhat decent fight but stood no chance - overrun by over forty undead, they were quickly surrounded and killed, although they took 16 walking corpses along with them. And since the necros had left early, those sixteen cadavres stayed down.
So, how to get back into control of the place? All we had were a bunch of civilians who were for the moment safe but without provisions and with no earthly hope to take care of more than one zombie - while we were facing 27. The provision problem could be solved by digging - the well-house is shut off with doors, so digging an access tunnel from the side could be done without opening a path for the undead. The siegers were also busy hunting down some yaks for a while, so opening and quickly securing an access to the food stockpile was possible as well. Digging produced a decent amount of loose stone, so the solution pretty much presented itself - a quick-and-dirty atomsmasher, hand-operated. Undead invaders path to your dwarfs, so they're pretty easy to direct, you just have to have your traps/installations in place before you open a path.
The result was satisfying, if not very glorious. I simply hadn't expected an incursion of undead, so was extremely unprepared. I guess taking on undead with military takes a larger force and defensive architecture that separates and channels the invaders a bit - being surrounded is troublesome against goblins, but utterly fatal against undead.