When Arkat came from the North and attacked Dorastor, the Cause was lost and many of the defenders of Castle Kartolin vanished, travelling far away or escaping into the mountains. The few who remained were cursed by Arkat to defend the Pass from all who would attempt to enter the land. Thus they became immortal Guardians.
Castle Kartolin is a mighty keep sitting astride the trade route into Dorastor. Its doors remained sealed until Ralzakark HeroQuested and proved several things to the Guardians. The gates were opened to allow some trade into Dorastor, but always at a price.
The Immortal Guardians are now few as many have been killed over the years by HeroQuestors and Adventurers. Their ranks have been swollen by mercenaries and adventurers looking for something new. They also have many of their ghostly comrades from the Bright Empire as extra guardians in times of war.
Castle Kartlolin itself appears to be a solid piece of stone, hewn directly from the Rockwoods. It is seamless with no windows or doors except for the main doors through the pass. It cannot be flown over as the Immortal Guardians have mighty weapons to use against flying creatures. It cannot be tunnelled under as the stone is too hard. It cannot be broken as it is said to be made of Truestone. Those who pass through its doors travel through a dark tunnel ten miles long and feel as though they are constantly watched. Walls cross the tunnel at regular intervals and Guardians test the travellers at each wall. Failure to pass each test means being turned away or even killed. The tests vary each time the travellers pass through. Entry to the Castle itself is not available through the Tunnel through its walls.
Only very special people may be permitted entry into Castle Kartolin. They must be Heroes or potential Heroes as those unworthy will not be considered. They may only enter at certain times and under certain conditions. Entry cannot be forced, it must either be earned or given. Entry is earned by HeroQuesting to prove worth. Entry is given at various times for unknown reasons (usually because the GM wants it).
Internally, Castle Kartolin is a never-ending series of mazes and rooms, many of which look out over the Pass but cannot be seen from the outside. This gives an incredible advantage for the defenders. Some of the rooms open up onto the Hero Plane, others onto the Spirit Plane or even the God Plane. Usually the Castle is empty save for the Immortal Guardians and their current companions. However, under special conditions and at certain times of the year, for instance at Sacred Time, the Castle will seem to be inhabited by those who went before. Thus, one can meet with the inhabitants during Nysalorís Reign, or during Arkat or Talorís attacks, one may even meet with the Heroes who guarded the Castle then - The Iron Vrok, for instance. This is a mixed blessing as these people may view the intruders as enemies or, even worse, as friends. Each time the Castle is visited it may be inhabited by different people, Humans, Elves, Trolls, Dragonewts, Giants, even Boggles. It is one of the Vanishing Castles, partially existing in the Mundane World, partially in the Other Worlds. Because of this, the players in my campaign would not have their characters set foot in the place as they knew that it was too dangerous. (Actually, that is not strictly true, they were flying over the Rockwoods when they were forced into cover by Monster Zero - I had watched a Japanese B Movie the night before and loved Monster Zero so much that it had to appear in Dorastor - and landed on Castle Kartolinís courtyard to be met by the Immortal Guardians and an army of Dragonewts. After talking with them for a while, the party silently vowed never to return to the place.)
For that reason, and that reason alone, I did no development of Castle Kartolin, which was a pity as I could have had so much fun winding the players up with ghosts and images from the past speaking of legends and prophecies.