Undead get triple stats and keep all their skill levels from life. Normal zombies aren't much of a threat because being killed causes them to drop all their equipment. Triple strength and legendary fighting still doesn't mean munch when you are nekid fighting a steel clad dorf. TOWER zombies (and thralls) tend to retain equipment. So you got skilled, triple strength, armored warriors immune to pain, drowning, and organ failure. That last one is huge, as piercing weapons rely almost entirely on damaging organs to stun/kill a creature (collapsed lunges, crushed brains, pierced heart are all one hit kills essentially) Warhammers are very strong (blunt) weapons, but do more organ damage (crushed skulls) than pulping. They are -very- dangerous without the necromancer reanimating any fallen corpses.
However... they can still be trapped and killed by cave ins. In particularly, falling items are extremely lethal. Dropping a worn out sock from a couple Z's can drop a Z :p
If they aren't encroaching on your fort, its because the fort is sealed off. Enemies will choose the shortest path. If there is no path... they won't tend to move. If you open your corridor they should shuffle in and fall to traps. Or you could create a cave in, or trap them in a pit. Bonus points- bolts don't damage zombies much, so you can use them as live archery targets once pitted!
As stated, maces do the most pulping damage. So while they normally suck, taking the time to shatter bones and mangle tissue will prevent the zombie from resurrecting. Silver is the best metal for blunt weapons, and I lke to pretend it deals more damage to undead. The only superior material would be platinum- but that takes an artifact made by a weaponsmith that likes platinum and maces to make without much chicanery (save scumming and/or forbidding all non-silver metal).