I've had some experience with this as well, though my numbers were usually 5-10 zombies.. And I've been doing just what you said, embarking with 10+ cages and making a drawbridge + gap for minor defense, but making the mechanisms and assigning the traps and bridge (and lever) (and channeling!) is pretty different than what I'm used to doing, which is digging a dorm-bedroom and basic storage areas while preparing beds/chairs/tables as a first move..
The early invasions are "dwarfly" but I'd prefer the initial necro-tower "probe" to be 2-4 zombies (still fully capable of destroying 7 untrained and unprepared dwarves, but more reasonable than 5-15..
Somewhat unrelated, but I have a fort of 80 that fought maybe 6 kobolds and assorted wildlife in its first couple years and then, out of nowhere 85 zombies show up :> I had heroic visions of my poorly dressed legendary squad of 7 methodically hacking 85 zombs to death in a blaze of glory, but they all got swarmed and murdered before dispatching even 10 zombies.. I like how the zombs shamble around all slow when they have no target but suddenly adopt panther-speed when they see a dwarf :> Right now I'm slowly trying to create secret above-ground fortifications to snipe at them from, but its hard to breach the surface safely when all 85 of them are spread out over the entire map.. If only I could set a surface fire..