This is amazing... the depth and detail are astounding. The color and contrast makes it leap off the page. I picture the sharp jagged rocks surrounding the place as the teeth of some ancient race of horrible creatures who were vanquished by the dark necromancer who lives inside. Great composition and flow... I noticed the small details of the fortress... tiny lights give it an almost futuristic quality. I know that in a critique you should give at least one point of criticism... but honestly... I would not change a single thing. I am looking forward to seeing more of your work. Bravo... instant favorite.
This is pretty high calibre work. This would be at home in some RPG book or something you'd see at a Con being sold, or like a Magic card thing. I always think of oils but you'd be hard pressed to get the detail. I only think of oils because you could sell a painting like for this for big money.
I think the only thing you need compositionally would be some sort of person going to that castle like some sort of hero or adventurer, not res'd in a high quality sense but an impression of it so we the viewer could become that person. I'm going there. I mean we are already there as a viewer.
Beyond that I don't really know, the palette is a bit limited but not really, like you couldn't really add to much to the palette. ?? Its hard to critique because its a finished, refined product.
I think what really kills the artwork in a good way is that you nailed the composition in terms of drawing the viewer in with the spiky mountains.