Instead of just making a bunch of cannons in one spot, make them organized this way. Make a pylon, then 4 cannons on the top, right, left and bottom, then place these 'clovers' accordingly. I have found them to be very effective against drops. And they save a lot of money VS a wall of cannons around your perimeter.

Mmm...butt-rubbin....Wait....wtf am I doing? God I am useless.