I really love the simplicitic appearance of this and how all the details are suggested by light and shadow rather than drawn in. [I did watch your video and it's amazing to see how you put this together.] The little touches of green are a nice contrast to the grayscale the rest of this piece is in. You have a wonderful way with depth and reflection on the water's surface. It takes some real talent so you have all my respect.
The horse's back legs seem a little thin toward the bottom. You get a real sense of movement with the flag and the cloth on the "warrior[?]".
It looks great, but the reflection of the moon isn't quite right. The greater 2/3rds of the moon itself is shown above the water/horizon line, so there should be an equal amount of it reflected on the water. By having the moon and its reflection as one big continuous circle, it creates this disorienting effect that the water is glass and you see the bottom of the moon through it. Unless that was the point