In all the talk of graphical downgrades no one seems much preoccupied with 'why?'. Why build something and then proceed to tear it down, piece by piece, in the hope that ever more diminished expectations about the final product won't be severe enough to...
Remember to make the lizards take the longest route you can when you put down your towers.
You can see the routes flying units take, so make sure they're covered.
Sometimes, it's worth upgrading old towers instead of buying new ones.