Bug: if you unlock towers then press Menu and retry, you get all your research points back but also the tower you've unlocked. So you can have all towers unlocked once you have 500 research points.
Level 3-6 is the easiest level by far! For those stuck on the 3 series, just build loads of lasers. The hardest is 3-3, you need to build all your lasers on one side so your alien can attack to the sides
 "Shop Empire Rampage"){kind=icon}
 "Dubious Defenders"){kind=icon}
 "Swarm Defender"){kind=icon}
 "Obliterate Everything"){kind=icon}
Yes, fewer invaders means less damage