Limbic Software has Tower Madness HD (iTunes Link) in the iTunes App Store for FREE (normally $7.99) for a limited time.

The highly anticipated, award-winning 3D tower defense strategy game has been redesigned for the iPad and now includes split-screen multiplayer! Evil aliens are coming to turn your beloved sheep into intergalactic scarves! Protect the flock using only quick thinking and the arsenal of upgradeable weaponry at your disposal.