Geography:
The citadel called Heart of Ice is near the northern pole of the world.  It is situated on a large, relatively flat plain that is in a basin formed by jagged mountains.  The mountains are very steep and there are few traversable passes to the interior basin.  The plain is a permafrost environment, with several feet of snow over a base of very deep ice.  The ice can reach a thickness of up to 200 feet.  As the lower layers of snow compact, they add to the layer of ice.  It is usually very windy, with the wind coming from variable direction, shifting quickly to swirl snow in blinding patterns.

Under the ice near the center of the plain is a large lace, completely covered in thick ice.  There are thermal vents on the bottom of the lake that warm the water slightly.  This and the insulation of the thick ice prevent the lake from freezing solid.   This warmth, however, is not nearly enough to melt the surface ice.

It is over this lake that the citadel stands.

History:
In ages past, the citadel known as Heart of Ice was built buy a long vanished race called the Mynar.  This race adapted quickly to any environment, allowing them to live easily in harsh climates.  The group that settled here and built the citadel was seeking escape from the political and social interactions of their culture.  They wanted only to concern themselves with their studies and meditations.  The citadel was built to reflect the stark beauty of the landscape.  At the time many now extinct animals lived on the ice plain.  Save for a few curious visitors from the south, the Ice People were left to their own solitude.

In time, however, the problems of the rest of the world intruded on the citadel in the form of a messenger bringing news of war.  A dark force had arisen, threatening to tear down the splendor of the world, and cast all to darkness.  The Ice People sent the messenger away, taking no heed, and lending no support.

Again, a messenger came, bear grim news of defeats and losses.  Still the Ice People refused to aid their kind against the dark threat, remaining aloof and claiming that the cause of the war must lie with the kinsmen to the south, therefore it should fall on their shoulders to remedy it.  The messenger left, wailing in despair, dreading the turn of the battle without aid from the north.

One last time a messenger came.  He was but a boy, for all others fought against the growing darkness.  The battle had turned, and darkness was being driven back, but at a terrible cost.  The land was sundered, the great cities in ruins.  The Mynar who had seen themselves as protectors of the land now sowed devastation in order to survive.  At the end of the battle, the boy said, the Mynar would leave this place, to ensure that such destruction would never again be wrought against the fair land by their hands.  This was decreed by the Elders, and to defy their choice was damnation.

With heavy hearts and guilty thoughts, the Ice People left their citadel and commanded, all save a few who even when faced with damnation could not leave the palace of Ice.  When the Others left, their souls were sundered, leaving them to wander the halls of the Heart of Ice as incorporeal forms, grieving for the mistakes of the past, with no hope of salvation in the future.

Present:
The citadel is an ancient structure built of ice blocks. The blocks have settled together over several millennia, almost completely removing any trace of the joints between the blocks. This has given the structure the appearance of being formed from a solid block of ice. Much of the ice is opaque, but there are some areas with crystal-clear windows. Even in areas without windows, most of the structure is well lit during sunny days. The citadel was constructed for creatures much larger than the size of a human, yet not large enough to comfortably accommodate giants. The vast fortress is sparsely furnished with finely carved ice. There seems to be no substance save ice.  The only organic materials that can be found are scraps of strange cloth and small pieces of finely-made paper. If the ice furnishings are removed, they will melt swiftly once outside of the citadel. 

The citadel extends far below the surface in a network of galleries and tunnels. The farther away from the surface, the darker it becomes.  These tunnels extend to an incredible depth, eventually opening onto a submerged lake.  The surface of the lake is approximately 20 feet from the bottom of the ice layer, and is navigable if vessels are available.  The water is bitterly cold.

Possibilities:
The ghosts of those who built the citadel still inhabit the halls.  If one can survive and find a way to communicate with the, they might impart vast knowledge.

An expedition is mounted to plunder the “Castle of Diamonds”.

Raiders mounted on strange, swift beasts have been attacking northern settlements, always arriving from the north.  All tracks lead in the direction of the citadel. 

Updated: Added history and more detail.