Impossible Fortress

At the heart of the labyrinth, safe from all save the insane, is the Impossible Fortress of Tzeentch. As with all of Tzeentch’s designs, the exact appearance of the Impossible Fortress varies according to the nature of the beholder's aspirations. Some perceive it to be crafted from the same crystal as the labyrinth, whilst others see walls of blue flame or gnarled azure stone. No matter the material, the physical structure of the Impossible Fortress is in constant flux. Spires and towers constantiy writhe and burst forth from the fortress' heart, only to collapse and be re-absorbed moments later. Gateways, windows and other portals appear in the eldritch building’s flanks, only to fold inwards once more. There is no discernible pattern to this behaviour, for the writhing shape of the Impossible Fortress is somehow bound to the state of Tzeentch’s current schemes and there can be no predicting such complexity.

The innards of the Impossible Fortress are no less confounding than the exterior. Different passages and rooms obey different physical laws. That which is decreed by gravity to be ‘up’ in one chamber may be ‘down’ in another; or can indeed be an alternate state of being entirely, such as sorrow or the past. Were a mortal to find himself in the Impossible Fortress he would not live long before being driven completely mad - but then, what else is to be expected in a place where a man can travel backwards in time by walking across a room? Those who succumb to the warping nature of Tzeentch’s palace collapse utterly in an implosion of consciousness and form. Such creatures are reborn as sorcerous familiars and given as gifts to Tzeentch's champions in the mortal world.

Even Daemons cannot easily endure the twisted horror of the Impossible Fortress — only the Lords of Change can safely navigate its corridors. As a result, no matter how distracted Tzeentch may be by the Great Game, he is never assailed in his stronghold. The other Chaos Gods have lost too many minions just trying to get beyond the first perplexing room, and invaders must negotiate a hundred or more such chambers to come before the Hidden Library and Tzeentch himself.

Source

 * Warhammer Armies: Daemons of Chaos (8th Edition) -- pg. 9