
Here I've drawn some guides based on the curve of the tower described by the roof above. As you can see, the bottoms of the windows should be curved to match. At the moment it looks like they're cut out of a flat surface, which doesn't look right.
The very tops of the arches of the windows should be touching the top line. This should help work out how the arches need to distort to wrap around that cylinder.

You have the lightsource coming from a bit to the left, but the darkest shade on your tower is both on the left and the right. In fact, you're going even darker on the left.
The second example I've added here gives you an alternative that makes more sense visually. It also gives more scope for using shade in the image later to differentiate different parts from each other.

In your image it looks like the door is standing proud of the building, which is visually confusing. See my top example, as seen from above. If you push it back to make it inset, the tower will describe a curve cutting through the lintel, as in the lower example I added. This would help the image make more sense.

As the lightsource is coming from the left, A should be darker than B.

Here, the flowers at B are in shadow, but also the same shade as those at A. The flowers at B should be darker.
Hopefully this is helpful. I'd love to see how you get on... If you're interested in improving this image further, let me know and I'll point out a few other areas to work on. If you want examples of the changes themselves, just ask.