Looks great for your first design!
I have a few suggestiosn though:
-The door seems to have more detail than the rest of the castle, so I suggest you have a consistant amount of detail throughout the gfx.
-Since it is flat design try not to put a lot of detail in it, try minimalistic designs.
-The background is alright, but instead of having a straight line between the ground and the sky I suggest you have some hills or mountains.
-Try fixing the small errors with color and placement of shapes.
As I said above it is really good for a first design, it's better than my first design.
I hope to see more of your work.
Keep these suggestions in mind for future projects.