The Next Floor

Three machine guns per floor will get you through round 9, and swapping one out for a laser will get you through round 10 and 11 without even having to click. End of round 11, swap out another set of guns for lasers. The only rounds you should need to click-fire on are 1 through 3, everything else your guns should be able to handle. Buy elevators as they become available. The start of day 14 you should be able to afford three lasers in every elevator, at which point all you really need to do is sit back and watch the explosions.

"we're sorry, the stairs are unavailable and the elevator only goes between these 3 floors, and these weird monsters are attacking every floor, please remain calm, chainguns and lazers are available for purchase in the gift shop. have a nice day!"

I hope this help a lot of people.
I found that if you max a turret and sell it you make more money that it cost.
So when u have the money to upgrade any tower to 5 do it, sell it, buy it, upgrade it, sell it, and again, and again.
I did the last level with all my turrets lvl 5(rockets and lasers)
+1 keep this comment alive.

I want to know who keeps sneaking in and re-attaching the lights

THIS IS IN ADDITION To Ozk4r123's comment:
On Feb. 24, 2014 he wrote:
I hope this help a lot of people. I found that if you max a turret and sell it you make more money that it cost. So when u have the money to upgrade any tower to 5 do it, sell it, buy it, upgrade it, sell it, and again, and again. I did the last level with all my turrets lvl 5(rockets and lasers) +1 keep this comment alive.

In addition:
To start this method u need at least $6.100
$100 dollar turret gives $450 extra (Need $6.100)
$400 dollar turret gives $600 extra (Need $12.400)
$1000 dollar turret gives $500 extra (Need 31.000)
$2000 dollar turret gives !$3000! extra (Need 62.000)
$4000 dollar turret gives !!$6000!! extra (Need 124.000)

There is logic in prices, not in money back guarantee xD