Welcome! I'm not exactly a environment design pro, but these are my thoughts...
- The thumbnail on the forum page tells me a lot...my eye is drawn appropriately towards the man castle, but it appears to be floating.
- The main tower appears a bit bent, going against the relatively straight nature of the rest of the architecture.
- The two joining towers seem overly flimsy and precarious.
- The tower on the far left is in a slightly different style to the main castle, and I find this slightly unsatisfying. Also, perhaps because it is on the same horizontal line as the main castle, it appears to be an equal distance away. Shouldn't it be darker? And more detailed? Perhaps bigger?
- Your image is too big for even my generous screen, making it difficult to make further crits on composition.
Hope this helps. Nice start, IMO. Lets see more (though a bit smaller .