With not much defense, the first waves come too soon and are too strong.
By sacrificing some planet health, it's easy to set up basic defense. Once that is present, it's easy to put up all 12 shields and build the 60 missiles. At that point, there is nothing more to do. Pretty unbalanced.
Why don't the shields regenerate (they only regenerate after being completely down, they should regenerate over time without the need to first fail completely).
My suggestions:
* Cheaper weapons at the start
* Different shield tiers (lower tiers much cheaper but easier to break)
* Shields could use planet's energy to regenerate
* weaker and fewer enemies at start, later waves can be more powerful
* Add the missing wave counter
* Hotkeys to select buildings, current UI design forces tedious mouse movement back and forth.