The makers of this game sadly hopped on the facebook-game wave and now spend all their time making Backyard Monsters and the Desktop Defender for facebook.
The red guys are heavily armored. This means that bullets are rather ineffective (which is good!). The electro-tower on the other hand totally destroys them. No remorse *grins evilly*
Just a tip to make it a lot easier.
This game would be great with challenges. There are so many possible.
Beat without using X type of planet, Beat X amount of levels at Y difficulty, beat challenge at insane diff, beat all challenges at insane... List goes on.
I would love to see some :)
 "No Time To Explain"){kind=icon}
 "Desktop Tower Defense 1.5"){kind=icon}
 "Castle Crusade"){kind=icon}
 "Frontline Defense Beta 2"){kind=icon}
 "StarBaron"){kind=icon}
 "Red Invasion"){kind=icon}
 "Nano War"){kind=icon}