I have a 0.42.02 fortress that embarked near a Tower. I got a dwarf (presumably a necromancer?) leading a small army of somewhere between 50 and 100 walking dead of various races (humans, elves, dwarves at least).
First thing I did was close the drawbridge, sealing myself in. This caused them to mostly clump together on the map edge, obliterating any visitors that came by. (And this fort got a lot of visitors of all kinds. Bards, scholars, mercenaries... didn't matter much. The zombies killed them all, rarely losing one of their own.)
I dug a tunnel under the surface and put in some green glass serrated disc traps. I wasn't bright enough to put drawbridges at BOTH ends of the line of traps though -- just at the near end. So, when I lured some zombies into the tunnel, and chopped a few up, and clogged up the traps, I couldn't send civilians in to clean the traps out. There was no way to secure the outer entrance.
So I dug another tunnel going the other way, and put drawbridges on both ends. I tried silver spiked balls this time. Lured some zombies in... and mostly they were not killed by the traps. A few were, but even more were able to walk back and forth over the line of traps, being bashed and smashed and so forth, but not dying. Tried a few iterations of that, but it just wasn't killing them often enough.
While all that was going on, I sent marksdwarves up onto the catwalk behind the fortifications, and took shots at them with silver bolts. Mostly these caused injuries but not deaths. I was hoping to get some lucky shots on the non-zombie leader. And he did take some damage, but not enough to kill him.
The marksdwarves turned out to be a disaster for my side. Eventually there was a body part on the catwalk that someone wanted to haul to a coffin. So, what happened was a civilian would go up there, get close to the body part, see the zombie, flip out, somehow end up on the ground outside the walls and become zombie food. This repeated, over and over, like 40 or 50 times. Civilians ending up on the wrong side of the fortifications was the major cause of death for me in this entire siege.
At some point the non-zombie leader ended up in a murky pool and drowned. After that, the zombies dispersed and started wandering more freely (instead of clumping around him). This didn't really change much, since even a single zombie could take out a visiting mercenary of any kind.
After a few more months, they left.