Good concept, but needs a few tweeks. A zoom (or anti-zoom) would be nice. As would a way to upgrade towers, or at least have some control over what's happening on the field.
It's unclear that 'ranged attack' is the characters special ability. Attack animations would be nice to know when characters are getting attacked.
otherwise all right
 "Bubble Typer"){kind=icon}
 "Asteroid Defender"){kind=icon}
 "Commander"){kind=icon}