So ... here we are. The controls are easy and good to handle. You should probably add some more ammo crates at the beginning and less at the end so it gets harder to have some ammo left the further you get in the game.
This is the only thing I can complain about, but I still got some suggestions for you.
Here they are:
1. Add some different specialities to the melee weapons, for example a slowing/freeze effect for the 2nd melee (units hit get slowed), or a burning/fire effect on the 3rd melee, setting enemies on
fire.
2. Add a chance to drop random packs of ammo at the beginning, decreasing the chance towards the end. You need more ammo boxes at the beginning than in the end. Probably give out more gold in the end to balance ammo/gold drops again.
Well, that's it for now. I hope this will help you.