Citadel

Citadels are the last vestiges of civilization. They are large, heavily overcrowded cities surrounded by reinforced walls of radiant steel, with scattered farmland or mining outposts nearby within walking distance or connected by railways. Upon the onset of undead swarms attacking the citadel, the surrounding areas are evacuated back into the citadel proper, and large armies are levied to defend the walls, repel the swarms, then take back the nearby farmland so that the huge populations can stay fed.
Most citadels were previously major capitals, industrial centers, or otherwise naturally defensible cities that survived the World Death. People fled to anywhere that could hold out, so these cities took on tens of thousands of refugees, regardless of species. The dwarves were luckier than most, and many of their fortified mountainhalls survived the world death and became citadels.
Over the decades since the world died, most citadels collapsed due to famine, plague, or undead invasion. The ones still around cling to stability on a razor edge, using every magical and technological advantage they can get their hands on to beat back a world that would rather see them fade away like the rest.