Original image. If you'd like to buy this print, you should do that from here.

Castle building 5/17:

This is just the castle towers themselves, no clouds or turrets or windows or flags. I used the pen tool for all of this. I found out just how important layers were - right now I have three layer groups set up: the original image, the background, and then one for the towers. I originally had four - background castles and then the front castles, but near the end, I realized I could combine them into one with no ill effects.
Adding turrets and windows and clouds will be next. I plan on making all of them using shapes to practice combining shapes together.
Final 5/19

The windows, flags and the turrets were easy - just basic shapes of triangles and rectangles, though I got a little fancy with the bottom left window and added a curved top to it.
The clouds were BY FAR my favorite thing to make - just a thin rounded rectangle base and circles in various sizes to create the puffy parts. I also added flagpoles to the flags because how else are those flags staying up there, am I right?
I just googled "grunge texture" and "scratchy texture" to find a couple of textures to put on top - this was actually my first time working with textures (I'm normally an editorial designer) so I might not have done it correctly, but I just placed it in the document, chose a mode (overlay for two of them, soft light for another) and then adjusted the opacity until it looked like I wanted it to look.
It doesn't look exactly like the photo, but it looks pretty close, and I'm happy with it!