All Undead buildings produce "blight" for a certain distance around themselves. All Undead buildings, except the Necropolis, can only be built on existing blight. This means that any expansion requires a Necropolis first. Starcraft players will be familiar with this process. Because of this requirement, the Undead are the only race that cannot Tower Rush.
There are two types of Undead worker units: the Acolytes, who can do everything but harvest lumber, and the Ghouls, who can only harvest lumber. Interestingly, the Ghouls are also the basic combat unit! Unique to the Undead, their workers do not need to do more than initiate the construction of a building. They can move on to other tasks while the building "summons" itself. This is also familiar to Starcraft players. When Acolytes pray for gold, they do not need to carry it back to a drop-off point. This means that they can mine gold faster than Humans or Orcs.