I want to see a screen shot of where your towers are when you get to level 81. I did get to level 77 using only green towers (and one blue to catch the ones that got away from the greens early on) and I tried putting 3 on the top row, 4 on the middle and 4 on the bottom row... Didn't quite make it that far, though.
Unfortunately I don't have that screenshot
But I made a screenshot of the placement and which towers to upgrade first.Tower Defence: Flash Game of the Day
The highlighted towers should be upgraded to level 4 before all other towers are upgraded so that they have sufficient range. After that, start upgrading all other towers gradually, including the highlighted towers. You may lose 5 or 6 citizens before wave 20, but the remaining citizens will last a long time.