The building itself stands out from the nebula that consumes the soul of the image. The astromancer's observatory is what draws me in most, because it's a modern touch to a medieval style of building, almost like a spire, for the main building.
As for the subject matter, it seems cut and dry. It's a traditional fantastic-looking starscape that fantasy artists have been drawing for years upon years. The darkness contrast given by the possible evil alignment of your astromancer does give a lovely play of shadow and darkness against the starry sky, but the originality lacks for the most part.
It pops. The picture explodes from the screen at first view, and there's no denying that. I hope that I give you a good idea of what I see. Obviously originality is the hardest thing to improve, since there has been so much done.
I didnt realize that "astromancer" was a term when I came up with it for a fiction piece I am writing, just out of Curiosity, what does it mean to you? Wondering if we came to the same conclusion. For me it was Astro=star, star=light, so Astromancer=Light manipulation magic