The speed painting looks good. The color balance looks a little flat though. You could try adjusting it to make the image pop more.
The building looks like it is leaning over to the right. The vertical perspective looks ok in general but what I think the problem is that some of the structures have thick bases and thin pillars and the bases taper to the right up the structure. This gives the impression of a diagonal and leads your eye that way. My first impression was that the towers were leaning, especially for the one in the far background. Additionally, the swooping motion of the flying buttress type structures also lead the eye in that manner. Something to think about.
One quick trick that you should try to help with assessing the value relationships in your images in photoshop is to create a layer above your image, fill it with 100% black and then set the layer to overlay. This will give you a nice grey scale view of your piece and makes a nice resource for judging value.
The composition is heavily weighted on the right side. Flip the canvas horizontally and you will see what I mean. Perhaps you could put another smaller structure on the left or put more into the environment to help balance the composition. When I flip the canvas horizontally, nothing stands out as a problem. I think that if you balance the composition a bit more you are good to go.
Lastly, the trees in the foreground. Right now they are 2 triangular blobs of green that are not doing much besides framing the bottom portion of the image. Break up the shapes and ref real hill sides. Make their design point to or compliment the castle in the corner. I did a paint over demonstrating the design shapes of the trees in red, I then drew some green arrows to indicate where there are some negative spaces which could indicate and point the viewers eye. And finally, I drew a blue line to demonstrate how the viewers eye should flow through the piece. Looking back, the paint over image might not be the clearest demonstration of what I am talking about but if you have any questions let me know. One thing to look at is check out your Loomis. In the book Creative Illustration, there is an excellent lecture on Design and composition in the first chapter. Check it out.
Anyways, these are my opinions and observations on your piece. I hope they are useful and I apologize for the point form answers but its been a long week.