The game should have a way to go straight to the end of the wave. That is, instead of waiting for all the enemies to cross the map, you can jump straight to the end result. That way, you don't force the player to wait until the wave has finished.
You should be allowed choose how you level up the heroes. The options should be speed, damage, or special abilities.
You shouldn't have to play the game so long to gain skills.
This game could work. I think the problem is that the player goes where you click when you press the mouse button. Hiding behind the wall or moving forward should happen when you release the mouse button. That will make it easier to do what you want to do.
 "Protector III"){kind=icon}
 "Incend"){kind=icon}