It was pretty hard placing the towers very close to each other. Sometimes you thought you place your mouse pointer correctly, and yet when you click, it does not get placed. And sometimes your hand moved a bit just before you click, and the tower is placed on the wrong place. And it will have block the place you want to put and that space is wasted.

Recently then I found out how to place correctly using the button. For the 1st time, I can place the cannons on every single available positions. It's still not enough to kill all the enemies in the last stage though. And I took my eye away for 3 seconds, and too many creeps escaped and I died.