Necropolis/Halls of the Dead/Black Citadel
The central base building. Produces Acolytes, the Undead worker units, although they cannot harvest lumber. Has a nasty defensive bite.
Provides food. Has a nasty defensive bite, although it isn't very durable.
Altar of Darkness
Provides upgrades. Continually spawns corpses around itself to be used by Meat Wagons and Necromancers and Ghouls. Also used as lumber drop-off.
Produces the early Undead combat units - Ghouls and Crypt Fiends.
Produces Abominations (big melee units) and Meat Wagons (artillery and corpse transports).
Temple of the Damned
Trains Necromancers and Banshees, the Undead spell-casters. Can upgrade their abilities here.
Used to convert Acolytes to Shades.
Trains Frost Wyrms (big honking air units). They might be big, but they can be killed very quickly by proper anti-air tactics. You can upgrade their breath attack here.