Location: The Winter Castle

So I felt pretty uncreative about this name. I actually don't even really like what I've done with this in general, even though I did like the theme. Unfortunately, I forgot to copy the name of the person who posted this image to Reddit. This may be a post that I revisit in the future, since I'm unsatisfied with how I've done with it today.


Description:
The air is frigid and dry, and the sky is overcast. The castle lies on a thin spire across a deep chasm, connected to land only with a narrow, winding path. The stone is yellow and thick, and the snow slides off it easily.

Associated Knowledges:
History:

• This castle's origins are shrouded in mystery.
• The castle is rumored to be of fey origin.
• The castle is known to disappear and reappear as the seasons change.

Geography:

• There are very few castles this far north.
• The mountainous terrain around the castle seems out of place.
• Even this far north, the cold seems more intense than it should.

DM Info:
The Winter Castle is owned by winter fey. It is inherently tuned to the season of winter and the magical existence of cold. It appears in the mortal plane briefly during the winter, and disappears after about a fortnight. It is located very far to the north, such that few people live nearby it's appearance. The fey who live there make a habit of not interacting with mortals, and will torture anyone who passes by with their cruel games. The fey who live there are not all evil or all good. They exist on their own system of morality, that is alien to most mortals. Some of the fey can be benevolent towards mortals while others are vicious and brutal.
The entrance into the material plane is not through any conscious effort of their own, but just as a consequence of their own attunement to winter. They are very powerful spellcasters, and are attuned to frost magic. Magic that is performed near or within the castle can become unintentionally transformed into frost magic. There may be no actual effect to this beyond a contextual re-skin of spells. 
The castle does not only exist as winter. As each season changes, the castle cycles between each of them and appears in the appropriate location in the material plane. Not all of them might be known to mortals. The fey that live inside the castle also change along with the seasons, with their personalities and abilities changing along with them. The winter castle is the only one that mortals currently have even a passing knowledge of, and even that is fairly rare.
Adventures into the winter castle would most likely be a rescue mission of sorts, or an extermination. Either a hapless villager was captured by the fey, and is their current source of amusement, or magical creatures have been terrorizing the locals. It would be filled with environmental challenges from the icy structure, as well as dangerous combat encounters from the fey creatures. Most of the powerful fey would likely not even be aware that the PCs had entered. This is similar to the last post in the sense that it has the potential to either be a one-off adventure or it could open the door into a much larger story. Or even be a continuation of that story arc.