WARNING LEVEL SPOILER!!!
Level 1 - Beginner Swarm
Level 2 - Diamond One
Level 3 - Dual Channel
Level 4 - Line Up
Level 5 - Down the Chute
Interestingly my hit counter has gone bananas since I posted the solution to GeoDefense Medium Level 3.
Obviously people are more interested in finding out how to beat this game than what I usually blather about.
The latest version of the iPhone firmware (3.1.2) crashed my phone and I had to do a restore, thereby losing all my saved games (the horror!).
Having to start again, I took screenshots of Easy levels 1-5.
I will post the other levels as I go along.
I make no claims to these being the optimum solutions. In fact looking at the leaderboards I can pretty much guarantee that they aren't. But posting these is more about showing folks that there is a solution. If you have any questions about a level, leave a comment here.