This is a beautiful shot. You have framed the castle to the left of the shot and putting it off centre makes it more attractive to the eye. The viewer has to search for the main focus, and this means the eye wanders over the image. The contrast between the green of the grass and the colours in the sky is also very effective at increasing focus on the main subject, the castle. You've captured an interesting sky wich adds much more to the image. A bland sky certainly wouldn't have been as effective.
If you look at the shape of the land and sea it's a classic left handed triangle composition which is certainly very pleasing. I think you've done a wonderful job, keep it up.