That was too short but that was fun ! The art is good and everything works well! The menu could get more polish (replacing the font would already make a huge difference) and going in build mode should be placed in a more accessible key (like Tab or Shift) but that's really nitpicking.
My biggest concerns is that the echo generator (which is a very interesting feature) doesn't actually have that much of an impact of the game. It basically just feels like the turrets shoot at regular interval which is a shame.
I'm sure with more time and more thoughts, this fun idea could turn into an incredible concept !