Very busy screen. If you ever place advertisements on there, it would personally be a huge turn off. Just looks too busy.
Play and pause button need re-designed. They take up a huge amount of screen real-estate. Condense pause into a single button that can both pause and unpause. Put it somewhere in a corner, probably condesed to only the two pause bars, small and out of the way.
As for the play button. Each level should have a button perhaps appear in the middle of the screen. Press that, it it is gone. Never to be seen again.
Difficulty. VERY and I stress VERY easy. Got to a point where I actually wondered if I could take damage
I understand I am "defending" my base or something. But this is not very clear. Not to make things more busy, but perhaps a better indication of what the bugs are trying to get to? Maybe the turrets have little people running around. loading ammo, shooting useless little guns off. small things.
I personally try to do away with health bars. They take up screen space, and usually dont do much. It might be interesteing, if you went the little people route, that as you take damage, the people die, and perhaps even the guns start to smoke and catch on fire.
As health goes up, turrets are repaired and people start showing up again.
All opinions. Do with them as you will.
Game looks promising. Keep up the good work!