Okay. Firstly: Welcome to the forums. I also assume you mean: "How would you describe a mansion...." Secondly, and more relevant to your question. I think some details might be needed here. what kind of mansion, what kind of snow-covered mountain? And other details like is there a forest? What is the mansion built out of? etc... In short, I do not think if I made a description it would be either appropriate or relevant. Besides, once you answered the question above, then you got your description. personally, I find trying to 'picture' it in my mind what the scene looks like helps when it comes to describing it.
"...a mansion in the mountains, blanketed by snow" comes to mind. Mountain-castle is a term but I don't think "mountain-mansion" is one. I do not think there are any specific terms for denoting a snow-covered-mountain-mansion and that what I put in quotes could be used to describe such a building after mentioning its name.
I think a mansion would be built into the mountain, not sitting in it. During winter months snow would camouflage the structure and only the reflections from the windows are visible.
Pictures always help to focus description. Study any of these that might fit your story, and see what is there. Also feel the cold, maybe smell the trees nearby. If there are wood fires going, they will also be smelled and perhaps even seen via smoke, etc. The weather will also matter. Is snow falling? Is there a blizzard? Is the sun shining? Are you able to see great distances, or is the mansion enclosed by the mountains and shut away from prying eyes, etc. Welcome to the forum! https://www.google.com/search?q=mountain+mansion+in+snow+photos&client=firefox-b&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwjd8K7YurLdAhVJDcAKHTm5Cp0QsAR6BAgGEAE&biw=1155&bih=787
First, a mansion implies a statement of wealth, power, or heritage to me. I envision it with grounds, statues, a large frontage with a gated entry. It may not be castle-esq. It could be understated, or opulent. A mansion has a butler, cook, and groundskeeper. So, snow covered or not, there's a need for land space, and some sort of dramatic presence from the approach. I don't see it tucked away - it wants attention. Location/orientation and access must attract the eye. It says look at me, lookit mee! Would it be blasted into a mountain - dramatic, powerful? Would it be an outcropping - bold, daring, adventurous? Or built out from the mountain on thousand foot pylons - eccentric, defiant? Or something entirely unique? Your call.
I don't know about the mansion, but snow covered mountains sound like hills like white elephants. Wait. Shoot. I think Hemingway beat you to it.
"A mansion was sitting in the snow-covered mountains" or did you mean how I will describe a mansion while I'm sitting in the snow-covered mountains? I'd be very quick in my description since sitting in a snow-covered mountain makes me quite cold.
