It's tower defense, if you know, and like tower defense games, then you'll like geoDefense, I won't go into detail about gameplay because it's pretty much a bog standard tower defense game, with a little twist.
That twist being a nice vector graphical style that copies Geometry Wars, with some very fast moving graphics and huge explosions, it does look nice for a WP7 game, although on some bigger levels with lots of towers, explosions and "creeps" things can start to slow down a bit.
The UI works reasonably well with the WP7 touchscreen, although it can sometimes be tricky to get the towers accurately lined up due to the screen size on my Trophy 7, also, as I am colour blind I struggle to see the "red" and "green" circle that surrounds the tower you are about to place to tell you if its possible to place it or not.
The game features a total of 38 levels, spread over 3 difficulties, easy, medium and hard, by default the game comes with the "hardcore" mode enabled, this makes the game very difficult in the later stages, but there is a "novice" mode that makes the game much easier, and it doesn't affect achievements either.
The achievements are very easy if you use "novice" mode (200 can be achieved in a couple of hours), but if you are intent on a challenge, stick it on "hardcore" and leave it there.
All in all, geoDefense is a very enjoyable tower defense game, and I think its worth the £2.49 entry fee, although some more levels, and difficulty level between "novice" and "hardcore" would have made sense.