It's really just a sub par tower defense game. The game has 9 levels, where you can get a maximum of 3 stars on each stage, based on how many lives you survive with. To be honest, there's only like 3 levels which are reused, just replacing the enemies in each of the 5 waves on the later levels. There are 4 tower types which can be upgraded to a max of level 3, which you actually upgrade on the start screen after earning gold from beating levels, then you can upgrade them in the actual levels.

Honestly, you can just use an autoclicker to kill the enemies, as clicking enemies also deals damage, and saves the time you'd be wasting trying to play this game normally.