The game have nice concept. I like graphic style, music and sounds. Only the GUI of upgrades can be more pixelated to fit the game layout. If you want to make it difficulty, you can add more enemies type or something like miniboss fight after every for example ten level. Also, it was nice to can buy upgrade and close upgrade window with keyboard. Most of the time player use keyboard (to move/attack) and only for upgrade he must use mouse.