OK so honestly, I really really like the basic idea of the game. Correct if I'm wrong, but on a fundamental level you wanto to create turrets to destroy the bugs but each turret you build has a certain "cost" associated to it in form of space taken. Build too much when you don't need it and you'll be in trouble later on.
If you've build something you don't like you can de-activate it in order to save it up for later. So you have the constant push & pull of having just enough to survive without constructing too much which can hinder you later.
The idea is very very good. You can do some very interesting things with this. And as a first version the game delivers. However you don't convey ANY information to the player.
Granted, I have only played 4 rounds of your game, but I still don't quite know what those two blocks on the top and the bottom of the trucks are. What do the different power-ups do? How can I obtain certain achievements? (Granted that last one doesn't really matter).
One change I'd like to see is that instead of having to hold down the mouse to build a certain cell, just let me select which tower I want to build up to and have it take time to build. I know that this would technically make a gameplay change because you can then build multiple things at the same time, but just make it that I can't build anything while another turret is beeing build. You can also either make it so that the player can/can't cancel/stop a build to increase challenge.
One thing I don't like at all is how certain bugs appear massive in size and only take a few bullets to kill. If a bug takes up 3 lines, it should take the combined efforts of 3 turrets to remove, not just a level 1 turret in one lane.
The OST is also a bit too much honestly, particularly for the initial levels. Something softer at the beginning would fair better.
You have a good idea here, but you need to explain your game better to the player. With a small tutorial on the mechanics and clearer indication on what is happening, you could have a cool game in the makings.