Awesome game! It took me a while to figure out how to pass the first "level" but I eventually did see the "e" popup when I hovered near the yellow tower. One way to ease the player into it might be a little arrow pointing down if the yellow tower is out of sight? I also was unable to trigger any of the towers in the second "level" to move on -- maybe that's intentional?
I love that fighting the little red pillboxes was a little difficult, I had to squish them into lining up and rounding corners tightly, so I could mow them down in a line. Fighting the red pills is pretty fun, and the explosion animation is really well done. I think the towers could be made satisfying in a similar way too. Nice work!