Really good work man! But here are some recommendations:
- Ability to repair the towers
- Fuse the carts to create better versions of them
- Other uses for Gold
- More cart variety
- When there are multiple waves coming in being able to control a second train maybe
- Give the building ability to create defenses over the breached ones
- Give a reason to sell things, sometimes is better to keep what you have than selling it
Overall a pretty good game