First, I'd like to say that I really like this piece, its pleasant to look at, and the amount of detail lets you continue to enjoy the piece for a while. However since you asked for a critique I'll write a review of some of the points that I think could be improved on this picture.
First, the rendering; its very impressive how good the water looks, and I think the rendering on the cliff face is better than you give it credit for. However, since you've rendered a high level of realism, some of the objects that you rendered less jump at me after I've looked at the image for a couple of minutes. The boat on the lower-right corner, the moss/plants on the cliff, and the port itself seem to jump at me a little more than they should. Maybe you could have used a steeper level of aerial perspective or different color choice for them?
The second large point that I noticed was the balcony at the left. Now, to be frank I didn't print this and make a perspective grid to make sure that the perspective was right, but for some reason it seems kind of off to me. For example, the rail seems to get shorter from the point where the man is to the point where the image is closest to the viewer.
I only have one other suggestion that I think may be helpful, towards the end of the canyon you lowered the rendering and simplified the color and shadows to give the feeling of aerial perspective, while its executed really well, I can't help but feel that the shadow of the columns may be a little too oversimplified, maybe a more complex shadow may add to that feel of realism.
About the stars, on this review, don't honestly think about them too much. I don't really think art can be graded so I just kind of went with my first instinct on them. I really did love this piece, and hope you get the chance to work on this kind of thing more often. For your first matte its hellof impressive and a great addition to your portfolio. If you need to scream at me over something I said, just howler, I'm usually friendly enough.