Looks like we're seeing a lot of Tower Defense games lately and here's yet-another-one for you - its called Enclosure. Enclosure is a twist on the classic Tower Defense game in which you not only place guns, but also build walls and doors to keep the attackers at bay.
Enemies come from all sides and have several potential exit points, all of which must be protected using a combination of force fields and heavy weaponry. The force fields are shaped into passages, rooms and obstacles, to slow the attackers down long enough for the weapons to kill them.
The game is FREE but has in-app/game purchases - the single weapon available in the free version can be upgraded twice for extra firepower. Additional weapon designs may be purchased in-game from Android Market. (You only pay for the ability to use the weapon, once purchased the weapon is forever available in the game).
If you're game on, check out Enclosure at the link below:
Enclosure (FREE with in-app/game purchases) - Android Market