Let me preface this post by saying that I do not consider myself a master builder by any stretch, but I can offer some suggestions.
First off, I think your interior is where this moc really shines. You have some fantastic features and the play value looks phenomenal. This reminds me of my first real castle build I did a couple of years back somewhat http://www.flickr.com/photos/fraslund/5 ... hotostream
(although there is much more to this one)
The one area I would critique would be the exterior walls. I personally love using panels as they offer a quick way to fill up space, but they are easy to overuse and can get repetitive. One way I have gotten around that is to recess them back a brick by surrounding them with support columns or in your case, you could extend your overhang all along the top two brick lengths at the top of your wall. This would create a depth that would combat that flat look. I don’t know if it’s the best example, but I used a good deal of panels in this tower. http://www.flickr.com/photos/fraslund/6 ... otostream/
My style has changed since then, but I think it worked for the build at the time.
The other thing that stands out to me is the dark gray accents. I like using dark grey as an accent color to force shadows or create an architectural break, but they can also be distracting as well. If you had the bottom layer of your wall all dark grey or just the inverted slopes along your parapets, I think that would look great.
If you look at these pics you can see that I used dark gray inverted slopes to create that shadow effect and even lined the bottom breaks of my central gatehouse breaks with dark grey plates. http://www.flickr.com/photos/fraslund/8 ... otostream/
Hope that helps,