First things first.
I confess that I just lost my first army in this current game in progress at beginning of the game which consisted of Morrigan, One skeleton spearman, and skeleton milita. To... Mites. I actually got myself besieged in my first village by darklings and mites surrounding it. I didn't leave until I have leather armor at least with four new spearmen built.
Darklings and Mites got routed shortly afterwards. But the shame of dying to mites once is eternal.
Then the way Undead grow their cities.... is quite different.
Don't worry about food yields, disregard them and only focus on production and essence values.
See that 1 food and 6 production tile? Plop a fortress right there and profit!
See that two essence tile? Drop a conclave right there and build the oracle!
Towns, eh, only build the towns if you need gold that badly.
Methods of growing the cities..
Constabularies. They increase ur growth by one for each constabularies you build.
Morrigan's Call requires you to have at minimum of 1 death shard under your control to start using this spell.
Various quests that give you buildings as Temple of assassins or Shrine of Martyr that gives growth and such.
And good old fashioned method of murdering humans from living factions and drag their corpses to your cities and convert them into skeletons :3.
Sadly, enemy sovereigns' corpses cannot be harvested.
To boost your population limit, you will need to build graves, they're like important, even skeletons need a place to rest their bones before acting again.
Mausoleum, catacombs, etc.
It's easy to grow undead cities if you can cast morrigan's call. But most of the times, you will have to do the hard work of building constabularies. They alone is only way to grow your undead villages if you have nothing else, including no humans to kill.
Then how undeads expand is really unusual. lol, normally with living factions you can just plop down a village and start growing right there. But not with undead, they need constabularies just to get started. And I noticed that undeads can expand safely if they do it defensively, not AI Style expansion like plop down a village in middle of angry monsters. And you gotta adopt that horde mentality. Sure it's nice to have a strong army, but, even when they're strong, they're fragile, its better to have several sacrificial armies on hand in case something powerful shows up and says hi to your brittle bones. (Plus, currently undead cities is defended by resoln militias. So you definitely don't wanna get attacked in cities.)
You should try to keep the battle to the fields, not the cities itself. If you thought skeleton soldiers was fragile.. you haven't seen how fragile the skeleton civilians is
As for the skeleton soldiers themselves..
They have this awesome terror ability and you can spam it every single turn without consequences. It's nice way to compensate for how weak the skeletons are. It casts afraid on enemies that fail to resist Terror and it last for a single action from enemy so they will be too afraid to do anything for their action but next action they won't be afraid. Good use of Terror will help you win battles that is unwinnable otherwise. And delay a losing battle for as long as possible as you attempt to do as much damage as possible before you get wiped out so that the chances of winning for next skeleton army is better than the former.
Another awesome thing is that Skeleton soldiers don't cost wages at all. Go ahead and make billions of skeletons if you want to, they won't cost u a single penny lol. Its just that they're pretty fragile and stuff.
Your fortresses will be churning out skeletons almost non-stop.
Even when you're expanding to fresh sector of the map, reinforcements is noticeably difficult for skeletons because they will need reinforcements from fortresses. Thus, when expanding into new areas, it's better for you to bring whole bunch of armies if you can to account for casualties that occur when you get oneshot or face drakes and even angry Trogs.
If it's like first 100 turns of the game, Yithril = Bowling Balls and Skeletons = Bowling Pins? Yeah I had that happen, I pissed yithril off and they sent several armies to bowl over my first fortress and I retired right after that. Oneshot, oneshot, oneshot,oneshot, oneshot, oneshot. Gee. Damn.
Simply put, my most favorite faction is Gilden, but I now have second favorite faction.. it's the risen undead faction. Polar opposites, i know.