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[?]".