I really like this, both the style and just the overall outcome of the process. If I had to give any constructive criticism, which I hate too since if you did anything differently it might not have come out so well, is that the castle seems plain. Structurally, it just doesn't feel like it has any depth nor are there any artistic architectural features to speak of.
I used a photo for a color palette and left some parts in the painting, like the mountain in the background. But for a painted texture, you can experiment with different brushes, or even create your own.