Who says it has to be just zombies? Don't night creatures such as vampires and suchlike technically count as being friendly to other undead such as necromancers and zombies
I would disagree for this form of collaboration, immortal beings such as necromancers and vampires would be (and should be) selfish against each other - wanting to be unchallenged, incarnate demi-gods of some sort.
It's not that the undead are 'friendly' towards the vampires, but they fail to see them as living creatures and don't attack (Similar to how zombies of rival necromancers don't attack each other but the rival necromancers themselves.)
As the undead are used for constructing a tower, I think that they are able to do more than just attack people, but maybe complicated mental tasks would be too much. Perhaps there could be other "classes" of undead in the civilization, that could do more complicated things. Like, if husks could come out of worldgen from people traveling through terrifying areas, then the husks could join the necromancer civilization as a general or something like that.
As the undead are already used to generate the towers (once the necromancer responsible has reached a certain limit) - it is arguable that the zombies have a degree of 'intelligence' within the aura of said responsible necromancer. Rather than having a specific 'race' of intelligent zombies, it would just make more sense for specific jobs to require the supervision of their master (such as the mining for stone and construction of towers) - similar to how a programmer creates an application for a computer to use. Mindless until programmed to do otherwise.
How about this: Necromancers are not inherently evil, but rather doing it "for science" (apprenticeses?), because they are "obsessed with their own mortality" (original slabbed necromancers), or maybe they want to see their loved ones. So my addition to this adea would be, to have different kinds of necromancers based on their personality:
- warmongering necromancers would build large armies of "low-tech", combat ready zombies that assault nearby settlements and thus grow their number. These are your standard necromancers and the kind we already have, essentially.
- peaceful necromancers would focus on perfecting re-animation and could return people back to life, not as undead but as warm-blooded mortals. However, they are unable to do it very often/quickly and thus do not rise armies.
- science-oriented necromancers could turn into Liches, extremely powerful, intelligent undead that come back to life back home even when their physical body is vaporized.
The necromancers would generally be on friendly terms. Most races would consider them enemies, because necromancy would be considered unethical (except maybe humans?). Whether a given necromancer (or necromancer society/school/tower/cabal) trades with you or tries to invade depends on their personality. It could even be arranged that they normally trade you, but can be pissed off in some strange manner (e.g. attacking or detaining any necromancer). It could even be arranged that they are pleased eith things that normally piss off other races (opposite ethics) but I don't think Toady would dare go that far into dark stuff.
I would disagree on the allocation of specific spheres such as 'warmonger', 'Peaceful' or even 'Scientific' - (Hey, I thought we were talking about magic here). The necromancers are sentient beings and therefore should behave in their own best interests. Naturally they would be selfish and extremely protective - armok forbid some rogue adventurer steal their much coveted immortality - and therefore attempt to hide themselves from the greedy fingers of locals. Truly, only the most boastful necromancers would dare to spread influence. Larger areas would require 'municipalities' to be formed under the close watch of necromancer apprentices - or 'competitors' in the strive for eternal life. Widespread interaction between necromancers and towns in the form of trade seems somewhat redundant. A necromancer with all the time in the world might as well supervise their minions in order to achieve their goals.
Furthermore, the concept of a 'undead civilisation' seems somewhat self contradicting. Civilisation being 'The removal from barbarianism' and 'The laws of criminal process' more so represent the development of an intelligent sentient race, rather than the meddling of a selfish, evil maniac. The concepts of secrecy and the formation of factions seems somewhat more fitting - like hidden pygmy tribes in the forest waging wars against other pygmy tribes.
tl;dr : I disagree!Edit: No more elves... or crude humour.