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.
Stumble This
I like the mood of it, with the techno music an vector-like graphics- but it doesn't quite have the simple elegance of DTTD. Too many options for my liking, maps and upgrade types. And does it ever end?
This is my current strategy, which was working fine until I got absolutely hammered by those 'dark' fuckers.