...Well, on second thought the idea to use undeads was not probably the smartest in the world.
I equipped our squad with full steel armor sets, gave silver war hammers instead of battle axes to them, and sent to kill undeads. To no avail: they all died after very long (longer than a month) combat with ridiculously modest results. I decided to roll back the game (I kinda suspected that there would be troubles, so I thought about this fight more as about an experiment).
I don't know why, but these undead heads are extremely dangerous enemies. They get no damage at all, dwarves fall in exhaustion very fast, and then heads eventually kill them.
Good time to think about alternative ways. In the worst case scenario, we can simply ignore undeads and unseal the entering - after all, almost everyone is undead now in Flamehorses. However, we would have to sacrifice all visitors, caravans, pets, few living dwarves, all future migrants. Doesn't sound fun, especially now, when we really need new migrants due to recent loss.
So, I tried to come up with another solution. Meet Volcanic Impulse Railgun!
The idea should be clear: potential enemies go on the narrow path, hanging over volcano, and our brave railgun operators can send presents to them, in the form of minecarts, accelerated by impulse ramps. I use gold minecarts as bullets for our railgun, since they are the heaviest ones we have (ideal would be platinum ones). We also can use lead, since we have tons of it, but it is much ligher and will do less damage. Railgun requires some tricks to use properly, you need to burrow a couple of dwarves to the operation room, plus you need to set up an alert for everyone, since otherwise they tend to try to go outside the fortress, using this path (with a quite predictable result).
The railgun almost guaranteed to drop enemies into the magma (note screening walls), in addition to harsh direct damage. Not sure if it work for such guys as the colossus though, they may be too heavy.
There are a lot of possible modifications, we can add more coils of path and separated railgun tracks for them. A squad of crossbows inside of the old library plus fortifications also can add certain effect. Finally, we can make additional tunnel with other funny traps, currently the path connected directly to the old library.
Long story short, it works!
All undeads, who tried to enter the alternative entrance, have been dropped into magma.
These heads are really hellish beings, they are not completely destroyed even after they are at the bottom of the volcano
They have lost all body parts, and now they are immortal ghosts of heads.
Now at least we have Plan B for a goblin siege or other troubles, which cannot be dealt with by our military.
Currently on the surface there is only one undead troll's head that accidentally rolled into the pond and seems to stuck, and one cat's head, which has fun fighting with a human warrior last two weeks or about.
The surface can be considered safe enough to reclaim it now. I also sealed both alternative entrance and railgun room, just in case. I haven't deconstructed the walls on the main entrance yet. May Armok help those who will put things in order on the surface, it is such a mess.
Some other notes, just as I was afraid, all our necros do not consume any alcohol at all, almost everyone is disgustingly sober now in Flamehorses.
They are terribly slow, especially obvious it was for deconstruction labors. A living dwarf make this two, if not three, times faster than a necro. Probably, it is a good idea to keep part of the fortress alive and drunk, for some urgent works.
We still do not have any new migrants, the game said twice "fortress did not attract migrants this season". I hope, it is only temporarily.
FPS troubles are pretty serious, but I'm not as good with dfhack magic as zazq is, so I left this to people who knows stuff.
The thief dropped the book on the edge of the map. There was a fire nearby, but apparently, books do not burn. At least, necromancy books. Maybe it is because it is binded in stone.
PLEASE, SECURE THE BOOK! We need it.
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