I like the small details in this game. Circle appearing randomly on the main menu, most stuff are made of circle like the font, score, projectile etc. Circle that comes out of the enemy made the projectile seems more impactful as well.
If I am correct, the currently only spawn more circle on later wave (feel free to correct me if I am wrong). I think a good idea is adding more variation of circle? i.e. Circle that will split into two smaller circle after player shoot at them etc... This achieves two purposes: 1) Player will be more interested to keep player to see what is on the later stage, 2) Player might have to prioritize what to shoot to have a better change of staying alive.
Good job overall!