Undead post, sorry :p
To those that keep asking this kind of question, i have managed to pop a LOT of towers in early world.
I described my dwarven search here more deeply :
http://www.bay12forums.com/smf/index.php?topic=154533.105But in short ->
The tricks is to permit some other races to have necromant. If not it does not permit a lot of apparition of Towers by the simple facts that if only human and dwarf make towers this lead to stick towers generation to "peacefull" world.
But if we want towers and undead, we do not really want peace, have we ? ^^
So we want to have more race to be necromant, and it is sticks to the gestion of secret. As far as i have understand the process, removing the MORTAL tag and making new secret with it doesn't do the trick (no elf necromant for example).
BUT, you can make some race mortal (just add [MAXAGE:XX:XX] to the creature that is immortal in vanilla. You absolutly can make them have VERY long lifetime if you want to keep praticly immortal goblins or elves for example).
But it does not do the tricks alone. In fact, by checking the legend mods i understood that there are necromant goblin created, but ONLY if others necromant (mostly humans) talk about their secret.
It make things difficult because it suppose that Goblins necromant and Human necromant can share common space, and in DF world it's rare. Together with the fact that necromant alone that try to reach others will be mostly killed in dangerous world, (mainly during the age of myth) make the generation of goblins necromant very hard with this setting alone, at least in dangerous ages like the age of myth.
At year 400 on several test, only 7 goblin necromant exist VS A LOT of goblins humans. Which give only few towers and one goblins towers... despite the fact that on these worlds there are in mean 95% of goblins :p
I dig a little and discovered that in fact a lot oh human necromant doesn't have themselves teaching necromancy by others but just discovered themselves by worshiping a deity.
And it make a "tilt" in my brain. I just check the "religion" of entity default and discovered that goblins is the only race with kobold to nopt have any religion (and yes, they do not even prey for their fun dominators :p )
So i just make that to the goblins, because he, they are evil no ?
:
[RELIGION:PANTHEON]
[RELIGION_SPHERE:CHAOS]
[RELIGION_SPHERE:DEATH]
[RELIGION_SPHERE:METALS]
[RELIGION_SPHERE:DEFORMITY]
[RELIGION_SPHERE:DEPRAVITY]
[RELIGION_SPHERE:DISEASE]
[RELIGION_SPHERE:WAR]
[RELIGION_SPHERE:CAVERNS]
[RELIGION_SPHERE:DARKNESS]
[RELIGION_SPHERE:MURDER]
[RELIGION_SPHERE:TORTURE]
Annnnnnnd, it works ! ! ! A lot !
In every game i generate with this i have now plenty of Tower very soon, near year 300 without problem and it is way more linked to the number of secret that before !
Well, this is my perfect recipe for more towers