I really liked the concept of increasing range and power of towers in close range, having the line render between them was a really nice touch. Overall had lots of fun playing :)
Some comments: I'd maybe include a scoring features/wave counter just to keep track of progress, would be nice to compare against. Also i think the balancing needs some work as the first few waves for a while were quite easy but then got really hard after i couldn't kill anything with everything on the screens. The music stopped playing after a short while too.
Otherwise I thought the difficulty curve was a really smart implementation in a quick way, and really liked the way you had to be smart about placement of towers so they don't fire all at the same time! Great job, also the graphics are cute ^.^