I absolutely love this game! It's really fun. However, I find level 5 to be a bit unbalanced. The first waves are really easy to defend but the last one is much more difficult! It's quite frustrating to play the first waves and die in the last wave, again and again.
Other than that, it really is a fun game. However, I think that a tutorial is absolutely necessary in the beginning as it took me a few minutes to figure everything out. And CPU usage is very high on Mac. Would love to see some CPU optimisation. I've had no bugs on Mojave.