Ok well, this is my first critique so i'll try to be as fair and objective as i can.

The overall feeling you give to whoever looks at this picture is a calm yet almost dangerous threat conjured by a fantasy-world castle. The overall idea is not extremely original but it's nonetheless quite interesting. I could easily imagine at a first glimpse battles between a powerful Mage and it's servants with intruders searching for the treasures it's castle holds.

Now for the aesthetic evaluation:
The castle is very well "melted" with the big rock which is a great example of photo-manipulation.
But other things end up taking away the great feeling of the image:
- if you look at the shadows of the rocks near the point of view, they are generated from a completely different light source. Toning down the stock image wasn't nearly enough to make it less visible and it takes away the realism that photo-manipulation aims to acquire.
- at the left of these rocks, the reflected light of the sun(in the water) has a different angle and position than the one the sun can naturally cast from that position.
-when you look just above the rocks, and in-between, the water is very calm, and even the position itself of the smaller rocks let's us think that the shore is quite near, but the castle is hit by turbulent waves and the effect you added to those waves hitting it, resemble very much that obtained when the water is very low. It's a contrast affects the overall realism of the image.
-The sun could have been replaced by a less defined sun, more blurry perhaps, to give a more calm and warm feeling (like "Devil-Grades" already gave example in the comments )
- the bigger rocks (the one you copy-pasted on the castle and in the left of the picture) have both a feathered white outline, one more that the other, that make them stand out from the clouds, and brings out the curiosity of the viewer to the point he realizes that they are but the same stock frame.

Well, that's about it i think.

So, to end it:

Vision: like i said, the idea was great, and i personally loved it...
Originality: ... but it's not that original.
Technique: several flaws that can be covered with more attention and work. A professional artist need lots of practice, after-all, to make a masterpiece.
Impact: great first impact on the viewer, but immediately reviewed after seeing the flaws almost too obvious in the foreground.

I hope i managed to be clear and objective enough in my critique. You obviously have potential, but you'll have to work hard on it and polish it.
So, keep up the good work and always remember! Have fun doing it, or else it ain't worth it