the interface killed it
This game got a lot easier (not better, though) when I figured out the interface: before you can buy one of the better turrets, you have to buy access to that turret type on the middle menu on the right - the red upgrades above 'gun,' which by the way is what you buy to upgrade you main weapon's strength.
At the start you only have access to S turrets, but once you buy the M upgrade, you can build and place M turrets. Then the red upgrade on the bottom right ups their power. The green 'Def,' not that I needed it, upgrades base HP while the red 25-100 above it heals that particular number of damage. I guess the only thing I didn't get was the T button in the top center.
I found it a little too challenging before all my turrets were out, and a little to easy once they were all in place. I agree that it would help to add waves between the enemies, and I think you could also add some more powers to the main gun, like area damage or a high damage special with a cooldown. Finally, you could add a button to lock the location of the turrets so you're not constantly picking them up as you're trying to hit the bad guys. Oh, and you should really, really think about autofire.