It's such a simple game but I've been sucked in.  I started using the "maze" strategy, thinking it should be the way to go, because it keeps the little baddies on the screen the longest.  I was never able to finish the medium difficulty setting though.

 This video below gave me the strategy I needed - I can finish the medium level every time, and have almost finished the "100" challenge.

   If you get tired of the simple scribbly graphics of the original Desktop Tower Defense- here's a knock-off  that has more realistic graphics, terrains and sounds.