Here is my strategy for the hard level 3 for the iPhone game GEO Defense.

• Start with the laser and vortex tower
• Begin by adding a single shock tower
• start adding missile towers around the bottom and right sides
• keep upgrading your vortex and laser towers when you get a chance
• When you run out of room start upgrading the missile towers

I might have gone overboard with all the missile towers and shock towers in the top-left section, but sure as heck nothing got through!