The three units work well together. They each have their advantages and drawbacks. It's so satisfying to see the cannon miss what it was aiming for only to destroy a few enemies that followed. A strategically placed cannon or ballista made all the difference.
Really appreciate the game speed setting. It works well too. The health bar wrapping around the screen is pretty neat. I can't say that I've seen that before.
I'd have liked to see numbers. How much damage to the towers deal. How much damage do the enemies have remaining.