Thanks!
Yeah I need to adjust the difficulty curve for sure. I also want to add more enemies, but art tends to be the limiting factor for me. I actually wanted to have animations for the enemy attacks, as well as totally different sprite for the turret upgrades, but didn't get a chance to do that.
I am having trouble running your game in browser and I'm on mac so I can't download. I know its me somehow because it looks like others have played your game in browser, so i'll try again later.