Undead hmm... Of course need to be scary! Dark foggy ruined village, swamps with frogs, dark old scary trees and yeah. The destruction will be the main arhitecture of the city's/village's! If there is more effects like spider, mouse, bat sounds will be epic!

I really would like something like "Zaun" from league of legends universe, they are kind of "Underground ruins" with a lot of "unorthodox science" for main city.

It's more like a city at the deep of the abyss. I think it fits the Undead theme.

Minor cities surely can be in any other dark places. Swamps, caves, Undead can live almost anywhere.

could see a church with fog and tombs around with several undead, would look like an ancient human city!

Greetiings

For the Undead I can think of 2 concepts.

1) Seeing the name of their queen, the Undead of this world could have a fantastic Egyptian style as Tomb Kings of Khemri or Vampire Counts (Warhammer).

2) Seeing that they are Undead because they rejected the Ancient Gods and were punished for it, they could have a Steam-punk style aesthetic. Why? They could be an enlightened culture, based on technology or archanotecnology now that they do not have the favor of the gods.

In any case, I don't think that because they are Undead, they want to live in cochambrous ruins or nauseating swamps. They could maintain a lifestyle similar to when they were alive, but themed with darker and gloomier colors.

See you

Something inside a ravine

Zaun-like architecture from lol