The undead city of Keter
It has stood the attacks of no less than 5 crusades. This is in large part because the very environment of the undead kingdom has took on properties of death. All the surrounding waters being filled with corpses and lake monsters. The very air being a poisoned miasma. The city then has a strengthened barrier that prevents intrusion both physically or through magic. The king has a direct contact with a portal to a hell which one is unknown.
The leader of the undead is the Named, Dead King . He has a great deal of power and was able to raise a whole kingdom of undead. His exact origin is unknown but he has ruled his empire with an iron fist for at least 5,000 years.
While he has not launched any attacks on creation many kingdoms have remained weary of the possibility. It is due to this connection that the surrounding environment is reinforced and he is able to maintain so many subjects.