There is still a lot of glitches in this game so beware. Some times the attack doesn't do anything at all. Enemies randomly spawn and you have to protect that giant blue thing on the left. If it gets hit three times, it's game over. You have to shoot them before they get there. You are immune to them anyway so don't worry. If you have enough money, click the attack speed+ sign to upgrade. The variable on the right is the price.
Credits: Youtube, also youtube, and my brain.