The castle would be entirely overcome by ice were it not for the heat from the forges which run throughout the winter. Its primary defense is that, for half the year at least, no enemy can survive the journey to it or the cold conditions that exist outside its walls. 
This is a personal project to work on my architecture design skills as well as brush up on my matte painting. 
See below for techniques used.
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To create the actual castle design required many stages. I did some blocking in with 3ds max, then brought those pieces into 3d coat, but some pieces were made directly in 3d coat. I would then add some wear and details and remove the hard-edged look of poly modeling. 

I also did some work in Oculus Medium for the rocks which I then brought back into 3d Coat to add detail.

Then I added materials/shaders inside 3d Coat and exported it all to Cinema 4D where I rendered it using Redshift. Here is the clay render from C4D...