Nice idea, a couple of notes.
I would change the color of the turret when it is upgraded (or at least give the information of the level of upgrading graphically).
The arrangement of turrets should be more organic, perhaps being able to put them on all sides of the map so as to add a strategic component, this also applies to enemy spawn.
Of course, it would also benefit to have turrets of different types, perhaps firing multiple shots at the same time or in bursts.
Probably moving the camera to a more top-down position would give better visibility of the battlefield.
In any case nice work, keep up the good work.