Nightmare Moon would be an evil Queen, not an evil Princess. I also imagine her having a very modular sleek castle, very opposite to Canterlot and Celestia. More science-fiction than fantasy... etc.
Several hours. Wacom tablet, Photoshop CS3. Process: [link]
EDIT: Fiddled with the shadows and the throne. Adjusted some lighting. Also, now in .png
Also, I lost the working file because I am an idiot, so I can't make any more changes on it. So while I love your critiques, please no more... because they'll just make me sad and frustrated because I won't be able to follow up on them... T____T