I've had a few Undead sieges with Necromancers now, I'm finding cage traps are fantastic.
They instantly neutralise the zombies (who seem totally useless at navigating through traps in general, makes sense) and avoid lopping off their limbs so you don't have to deal with creating more of them either.
Additionally, Necromancers themselves seem to blunder right into them, I've caught 3 so far and killed another. Apparently mastery of the arcane arts doesn't really do much for your peripheral vision
My advice when killing them is to sic your army on them once the corpses are thinned out (to keep your military from suffering needless losses). They have a really annoying tendency to run away like little girls, go off map, then come back again with loads more corpses. So Marksdwarves are great at nailing them as they flee, allowing your other guys to catch up and maim them.
The last one I caught I decided to have my Wrestlers fight for experience. Unfortunately I forgot to lock the door on the cell, and he escaped, reanimated a dead donkey and then proceeded to sprint into the dining room. At that point he copped a Crossbow bolt in the lungs and got set upon by pretty much everyone who was eating a meal in the room at the time, with the predictable result