There are a few ideas to dig out of the vault regarding the nature of the undead living beneath the benighted city of Lichtstadt: Hellish Ghosts and Demonic Corpses, and More Hellish Ghosts and Demonic Corpses. The ideas there are pretty simple; in the old AD&D paradigm, there are Lawful Evil undead that are associated with Hell, and Chaotic Evil undead that are associated with the Abyss. Summarizing the ideas from those earlier posts, wraiths, ghosts, spectres, and similar spirit undead are lost souls consigned by the power of Hell to guard or haunt a locale; ghouls, ghasts, and vampires are all physical undead cursed to spread death and destruction.
Once these relationships are established (and don't take my word for it - they're right there in the 1E Monster Manual in the demon and devil sections) it becomes easier to place the different undead in the setting, and tweak their powers to fit the themes a bit stronger.
Spirit undead attack the soul, and their primary weapons are soul-blasting fear and energy drain attacks. Corporeal undead destroy the body, eating and tearing the flesh, draining blood, and leaching away physical vitality and strength.
I've already used ghouls as the backbone of a zombie apocalypse (back in the Gothic Greyhawk campaign) so I'm going to take them in a different direction for Lichtstadt. Vampires will lose their energy drain attacks and rely solely on blood drinking. The demonic alignment gives me some ideas to mull regarding what lies in the depths of Lichtstadt Dungeon; perhaps there's a demon prison deep in the dungeon, or a portal to the Abyss; maybe the ancient progenitors of the different vampire bloodlines have different demonic patrons (and thus different vampiric powers) and the sleeping forebears of the lineages are deep in the earth (like the World of Darkness "Antediluvians"); it gives me a lot with which to work.
Factions and allegiances are important tools in the megadungeon for the DM to provide opportunities for politics, layers of intrigue, and strategic options for the players; just this small amount of background above suggests many factions among the undead. At the cosmological layer, you have demons that wish to destroy the mortal world and wipe out life, and devils that wish to corrupt and control the world. Then the vampires themselves will be organized into competing lineages or bloodlines (perhaps with competing demonic patrons); within the vampire lineages, there are groups that want to maintain as much secrecy as possible, versus those that want to rule openly as lords of the night, treating humanity as prey and cattle. There might even be a few rogues seeking to break their vampiric curse or discover how to regain their human souls.
Besides the strong Law and Chaos aligned undead, there are "The Stitched" - golem-like constructs built through forbidden alchemy. I like the idea of some nasty free-willed Stitched, genius-level monsters like Frankenstein's Creature, engaging in their own grave-robbing to gather loose parts to make stranger and more gruesome aberrations as footsoldiers and champions to prevent their original masters from ever regaining control of their wayward creations. Lichtstadt Dungeon is going to be an interesting place.