When most of the cities are covered by a terrible curse and some of the inhabitants turn into undead, you can retreat, saving yourself. You can gather the surviving residents in the surviving cities, you can wait, recover and lick your wounds. It’s possible. But you can’t retreat further. You cannot hide in the furthest hole to dig a new one and retreat there if the undead press again - this is not human, it is wrong. People were driven out of their inhabited lands, which means they need to get theirs back. Check if the faithful golems have rusted, Architect, and join the campaign against the necropolises captured by the undead. And let the liches look at you powerlessly, the vampires moan and gnash their teeth. Let the banshees howl helplessly and the earth burn under the feet of the retreating undead - after all, you are coming! Your army of steel is marching. Humanity is advancing.