Well, I just finished the game, including Rhythm Mode. It was really fun! The concept is interesting, the enemies are diverse (Capykaze is my favorite), the gameplay doesn't get boring after the first round (even though I expected it to), and I really enjoyed the music. But I'd like to go over the problems that prevent this game from being perfect to me:
1) It's really hard to tell when the beam attack recharges. Yes, it recharges every 10(?) kills, but during intense gameplay i'd like to have clearer indicators. The recharge sound is too quiet (especially when there are a lot of enemies), and the visual indicator blends in with the color of the light from the tower's eye. I didn't even notice it on my first playthrough, I was constantly looking for some kind of visual sign.
2) I REALLY love the idea of Rhythm Mode, and I was very excited when I got it. But the mode has two big problems: the first one is that the rhythm is tied to an in-game timer, not a delta/music timer. Any lag / Game Pausing / slow HTML running completely disrupts the rhythm, because the rhythm doesn't try to match the music, it plays separately by the timer, and after few seconds Rhythm Mode turns into a madness. The second problem of this mode: although there is a visual indicator, it's almost invisible due to the light from the tower's eye like it was with the beam recharge indicator. It may sound comical, but to play Rhythm Mode comfortably, I had to change the tower's eye color to black (making it completely invisible) and... turn off the music :D I simply stared at the indicator and clicked along with my inner metronome.
Aside from the issues, I really enjoyed the game. I played both the HTML and Windows versions, and I really wish the rhythm in Rhythm Mode would work properly, because i really liked this mode!
Oh, and yeah, my personal record in Rhythm Mode is 4501959000