I kinda like the main conceit of this game - controlling the windmill is fun. My main quibbles have to do with completeness and sticking to the resolution requirements. You really need sound or music in a game like this. You're also rendering subpixels on the grain and sheep is not locked to the pixel grid. It's pretty hard to tell the difference between facing forward and backward - a few frames of animation between each direction would go a long way to making the control feel smooth. When sheep come from the top or bottom, you have almost no time to react.
You have a fun idea here and it would be worth expanding on.