Mechanics are sound. Visuals could be bettered by ensuring more consistency. As a prototype it seems OK.

Edit in response: In my opinion, visuals alone wouldn't move your score much but your other planned changes might, particularly AI. Most of your audience here will be playing single player. Regarding the visuals, my issue is the inconsistency stands out. For example, if you look at the pixel density, the difference between the tanks and the walls is quite jarring. I believe the lower density pixelated style seen in your walls might complement your game better due to the simplicity of the idea. I would then match the UI text to the same pixelated look. But this point is somewhat moot as I think the AI would be a priority to tackle. Again, for a prototype what you have so far is fine. I can't offer up any advice as to whether to complete this game or to tackle more prototypes - that's entirely up to you.