As in most TD games, you'll fend off an ever-increasing army of creeps by strategically placing different towers to defend your homebase. Your towers have a range of attacks from a standard shot to a missile with splash damage, and you'll have to pick the right one to deal with the range of enemies, which vary in strength and speed.
What really sets this TD game apart (besides its visuals, we'll get to those in a moment) is that geoDefense is hard. We're talking "try the same level 10 times, throw your iPhone at a wall, buy a new iPhone and finally figure it out." That sort of hard. The field of play seems smaller than your typical TD game, so when to upgrade and when to build becomes a matter of split-second timing as much as it is a strategic choice. It can be frustrating, but it also helps to lift geoDefense above the countless other TD games.
What also help are the visuals which, no exaggeration, are Geometry Wars. If GW was a tower defense game it would look exactly like this. Uninspired? Maybe. But it's also a treat to look at and the simple shapes help the phone's screen from seeming over-crowded.
Perhaps the very best selling point for geoDefense is that you can try it out for free and see if the the frustration is too much before you take the purchasing plunge. Just be aware, that $0.99 is a lot less attractive when you factor in the cost for the iPhone you're going to snap over your knee.
geoDefense Lite (Free, Critical Thought Games):
geoDefense ($0.99, Critical Thought Games):