Pretty decent tower defense! One problem I see with it is that when you position your ship on the track and one of the aliens touches you, you can become immobilized for a really long time (as you get out of the stun, you get hit by another alien, and it continues until the track clears). It might be good to give the player a few seconds of invulnerability after recovering. Other than that, a really solid entry. I like it. ;)