Absolutely incredible! You took familiar mechanics and turned them into a difficult puzzle game! I was determined to get past an entire wave, and somehow managed too? I almost got to level 9 >-<
I think if you eased up on initial difficulty, added some other kinds of enemy units, and designed some specific puzzles in the form of alien wave compositions, you could have a really nice, full puzzle game.
Your attention to detail, like the little sounds that happen when you hover over a button, and the little stopwatch animation when it's not your turn, are really incredible and inspiring. The amount of polish here is excellent! And shooting through bullets? Super satisfying.
My only gripe is that it seems like the game is a little forgiving with the aliens changing direction. It is my personal opinion that the aliens should have to start their turn with a unit against the wall in order to move down. I believe that thet currently will move down if they ended their last turn with a unit against the wall.
Great job! Definitely one of my favorite entries!