It's a good game, try to improve the graphics. You can make enemies die faster, or give more money when you kill them. Clicking for half an hour on the screen for just one upgrade is annoying and can be improved, for example, by implementing a bonus that gives you x2 money if you buy it. It would be good if the enemies increase their speed and number of enemies, try to make the game more fun in that aspect of progression. If not otherwise, after two minutes playing you have already seen everything there is to do.