Suppose it's better to update enemy's HP, maybe you should add more enemy's types ( like tanks, armoured trucks etc.)
Keep updating, you're doing great!
Idea - great.
For the beta, execution good.
1) think about money growing process - 25 for each battle is not enough. Possibly reward should depend on the enemy's level ?
2) add tooltips for player
3) give an ability to upgrade ship
4) find a designer to draw some pictures
5) continue working over the game - and it'll be great one
 "Simple Tower Defence"){kind=icon}
 "Rebuild 2"){kind=icon}
 "Protowing"){kind=icon}
 "Survivosaur"){kind=icon}
 "My Pet Protector"){kind=icon}
 "Epsilon Strain"){kind=icon}
 "Modern Warfare 2 BETA"){kind=icon}
 "Star Trek"){kind=icon}
 "SunDog 2: EARTHSHOCK"){kind=icon}
 "Snow Virus TD"){kind=icon}
 "Space War Odissey"){kind=icon}
 "Farm Mania"){kind=icon}
 "Zombie Empire"){kind=icon}
 "Frontline Defense First Assault"){kind=icon}
 "Tactics 100 Live"){kind=icon}
 "Frontline Defense Beta 2"){kind=icon}
 "Destroy the convoy"){kind=icon}