Interesting take on the tower defense genre. I liked the presentation, in particular the various explosions/weapon particle effects. However there was no sound or music (is that normal?).
I felt like there wasn't much strategy required because you can always go back to the base to heal... so all you need to do is head out, clean up some towers, then head back to the base to heal & repeat the process. My suggestion would be to make the levels much smaller (less towers) and remove the healing, though that would be quite a radical change :) I did enjoy playing the game though & good luck finishing it!