I really liked the fact that there are upgrades and bosses. That gave a real sense of progression in the game and make the game much better than it otherwise would.
One thing I would improve is to make progression at the beginning faster, such as : make the bosses spawn earlier. The beginning was a bit tedious and slow.
Another thing is for exit to have a confirmation. I clicked shop then clicked exit , thinking that I was gonna exit the shop. Instead it exited the game.