Here's the deal about stacking: u stack cannons so more of them shoot, and you don't lose the game by the 2nd level (high templars). But with these Defense maps, pros (like me

) don't stack too much, we make
mazes (unlike some newbs that just stack 20 cannons in the center of their squares) so that the critters have to go a longer distance, during which they
die! Stacking also eliminates the space, e.g. between pylons so the little zerglings or probes have to walk around as well as big ol' ultras. However, they never thought of simply putting a small unit with range (e.g. marine) inside that space, which practically acts as a cannon without the stacking.
Thats true... But what are going to do about missons that you must kill flying things. You cant make mazes for them. And what if you run out of space for making turrets or whatever... Almost every Defence map in Staredit has the option of stacking...