This is one of the hardest levels in my opinion but that makes it one of the most enjoyable. I play it a lot and this is one solution that works, at least on iPhone 3G. Sometimes it gets a bit close to the mark on level 20 but once you have got past there you are home and dry.
I link the blasters and lasers to the Vortex Tower and after level 20 get the Vortex to level 7. Don't bother linking the missile towers.
I link the blasters and lasers to the Vortex Tower and after level 20 get the Vortex to level 7. Don't bother linking the missile towers.
Before you get to Level 20 make sure you have at least one Shock Tower up to level 7 near the end otherwise some will slip through. Also have a Blaster Tower at level 7 near the end so it only fires when the critters get close to the exit.