Waste of potential
I like defense games. I like them a lot. One of the most important factors in game development generally is: balance.
Many complained that it's sufficient to use the most basic and cheap tower. Balance the towers out, make the other ones useful. Maybe even unique. You have to make the player being forced to use all of the towers. Maybe the one pierces armor while others don't? Maybe one is able to poison the enemy just like the trap? Another tower could focus on air - units, if implemented. Use spread damage and a proper projectile/explosion animation. Make the ground units more useful. I didn't use them once. What's especially important is that the player notices the changes when he upgrades a tower. I love defense games simply because you can develop your weapons and make them cooler and stronger. Change the color with every step of upgrade. Change the general design of the turret. I didn't upgrade throughout the whole game because it's far cheaper to just buy another tower of the same type - change this. Make upgrades more powerful, alter the projectiles a bit, make the tower look mightier with every upgrade.
Don't make the enemy charge right at your spaceship. Some of them should move on another path. And don't make them get so close to the turret. At least not every unit. Looks bad. Add pause.
Sorry that I'm criticizing so much. The game's really big and you apparently put a lot of effort in it, but isn't it a shame that most features remain untouched. Your wasting much potential. Try to change the balance, add better upgrades, make troops usefuller and work on the graphics and it will be a hit.