A competently executed defense shooter, with a unified aesthetic that's pleasant enough. The music and sound start to grate on both the ear and the nerves after a while, but that's forgivable.
My big suggestion would be to make your "hard" mode an option right out of the gate because the first playthrough (as has been said) is startlingly easy. I played the entire game, levels one through twenty, with nothing but the starting weapon, fully upgraded. I literally never touched the keyboard- no switching weapons, no using "powerups." That by itself is enough to say that your upgrade system needs to be better tethered to the math that underlies the shooting gameplay, but that fact that I was able to do the above while NEVER TAKING A SINGLE HIT is what takes this from 'slightly thoughtless design" to "actively insulting the player."
In a game like this, the durability, numbers, grouping, and defensive abilities of the enemy waves should drive the player's spending, with the potential damage and skill-based attributes of the new weapons or upgrades keeping them JUST BARELY ahead of the rising curve. The bad guys should do at least a little damage, or maybe just make it to the front door basically every round. That kind of tension is what makes a defense shooter engaging, and the only moment I experienced it here was during the big group of the undead giant things at the end of wave nineteen, and even that wasn't the tension of potentially losing control, but of maybe having a single black mark on the no-hit pistol-only game I had going.
tl;dr? Not bad, fairly entertaining, but could have been much more so with some actual challenge.