A lot of interesting ideas in this thread, so thanks guys. It's a shame I won't be able to use everything, since a lot of it makes up for very interesting encounters. Human shields won't really work story-wise, but I am not going to reveal the details since there's a chance, however small, that one of my players comes across this topic and I don't really want to spoil too much if that's the case (so I'm a bit restrictive on giving out too much info).
I'm pretty much set on my original idea of using mounted undead at this point, and I have created some special skeletons with intelligence 3 (meaning I can put a few points in ride) and a few feats which should allow them to ride effectively.
Regarding spectrals, weak ghosts should do the trick I believe (or stronger if I want to have just 1 or so).
I've considered using the necromancers as pure summoners, "healers" and debuffers, and make up for the lack of damaging spells through the use of some spellstiched undead which I can put on the wall. Most of the spellcasters I've put against them at this point have been nukers, so having something that isn't a nuker in any way should spice up the encounter a bit (and there's plenty of things that can do damage already).
Regarding magic jar, I'm wondering if this tactic isn't going to be a bit too devastating. If the orc barbarian gets possessed he can easily kill the rest of the party if he uses all he's got, especially with cavalry around to help. Trying to keep it somewhat winnable here, which is always a challenge since I roll openly.
Regarding death knights, I looked them up and decided not to use them. Partly because they fit in badly with the story, and partly because they feel like they're slightly too hardcore if I'm to have more than just one. I decided to make the leader of the skeletal knights into a dark elf instead.
Regarding desecrated ground, I'm really considering that idea. Maybe have something like a crystal that gives turning resistance to undead within a certain radius from it, and that will have to be broken in order to bypass this.
So, what I'm considering so far is:
* A few special skeletons with int 3, mounted combat and ride-by attack feats and some turning resistance mounted on skeletal heavy warhorses. Skeletons wielding lances and both skeletons and warhorses heavily armoured.
* Skeleton archers on top of the walls. Undecided how many, but these are weak as hell and turning them easily destroys them.
* 1 or 2 spellstiched undead of some type. Maybe mummies or ghasts.
* 2 weaker casters and 1 stronger one, dark elves. Weaker ones are probably going to be clerics while the stronger one will probably be a wizard or a sorcerer.
* 12 mercs all in all, half have reach weapons, all have crossbows they can use instead of their melee weapons. Level 4 warriors split between dark elves and half-drow (yeah, they actually have darkvision and not low-light).
* 1 mercenary leader. PC-classed dark elf (probably a fighter). Rides out with mounted skeletons on a nightmare.
* Spectrals, type and amount still undecided.
* Something made specifically to watch the inside of the cave and which will not leave it to join the fight. Has to be appropriately mindless. Considering 1 or 2 flesh golems.
* Some worker zombies, but these will not join the combat in any meaningful way except maybe soak up turnings.
* Maybe some fighter zombies that are little more than an annoyance or just there to soak up turnings, and which mostly exist for story reasons.
* 1 dark elf rogue with some magical gear. Leads the archers.
* 1 crystal or altar somewhere that grants turning resistance to all undead within a certain radius of it until destroyed.