The undead city of Keter, thanks to the structural changes inspired by the multiple raids into the city launched by Ranger, as well as measures of separation by other races such as the dwarves sealing up the tunnels between their civilizations, and the undead that call the Kingdom their home, the city is near impregnable.
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 taken on aspects of death; all of the surrounding waters are filled with corpses and lake monsters, the very air is a poisonous miasma, and the city itself has a strengthened barrier that prevents intrusion both physically or through magic. The city is also host to a Greater Breach, a stable and permanent gate into one of the Hells, though which Hell specifically is unknown.
The leader and sole independent occupant is the Named Dead King. He is noted to be one of the greatest sorcerers to ever live, and was capable of turning himself and an entire nation into undead, through which he has ruled the region for something approximating five thousand years.
While he has not launched any attacks on the rest of Calernia in any living memory, many kingdoms have remained weary of the possibility, due to the sheer number of subjects he has and the power he may personally exhibit.