Something I've been wondering about. Evil biomes will continuously reanimate the fallen, yes?
And they don't re-animate if the corpse is in a different, non-evil part of your map?
So, what if you could use the corpses as a defense feature... a very lame one, probably, but still.
1) Keep a bunch of corpses, hair, shells, dismembered body parts, etc. in the non-evil part of your map.
2) When a siege hits, use a remotely-triggered system to move all of the body parts into the evil zone.
3) Allow your undead army to draw fire from sieging enemies.
Step two is the sciency part... how to move corpses from point a to point b when point b has enemies nearby? Some ideas...
-keep the corpses in an overground reservoir of some kind, with a removable/openable bottom.
-- Have several of these, so that they can be dispatched to various sectors of the map as needed.
-utilize water power: release a flow of water when desired, pushing undrownable corpses into the combat zone.
-bridge-a-pults... I forget, do those still work...? It would be an awe-inspiring picture though. The Goblin hordes walk towards the fortress, when suddenly a bunch of hair and shells and bones and crap gets tossed at them... and then suddenly comes to life and starts attacking.
-cage traps attached to levers will do something equivalent, but isn't very scientific...
-create ground-level rooms with doors/floodgates attached to levers. Dump corpses/etc. into them from above (use one level of stairs), unleash the horde when the other horde shows up.
And remember, the more dangerous the undead you release, the better. Apparently undead elephants are flippin' fierce...