I have played dozens of games like this over the years and seen many times more. There is nothing distinguishing this game from the others of the genre. Nothing there is absolutely no new or creative improvement.
Upgrades were completely meaningless. Even after buying all turrets and most units the same strategy that worked in the first level still works. Make as many solar panels as possible. Spam one cheap unit enough for it to get through the enemy turrets. The only difference to the first level is that I'm now using P90s instead of Glocks and Eagles.
Summary:
Very old and used game idea: 2 stars basis.
Absolutely no improvements or engaging creativity: - 1 star.
Upgrades are completely useless, rendering the whole came utterly boring: -1 star
Constructive Criticism:
Make the Missile Launcher and Ultra Blast more effective. At the moment they are not worth building. Give the player stats for all units in order to allow informed planning. Make the units more different. Include area of attack units, for example. Include different terrains, it's boring to visually and strategy-wise play the same level over and over again.