I found that the best way to make this game work is to build a tube around the path the attackers take.
Since their logic forces them not to think ahead, they are easily killed before its too late. Later in the game, its how slow you can get them while killing them off that is the trick, so adding an alternate slowing element, with rocket towers works fine.
Don’t bother with anything other than rocket towers for destruction. Don’t bother raising the heights of towers for more range, its a waste and slows the response of the towers.
Build starting from your cookie fortress and work your way out, toward the path of advancing attackers. Below is a view of what works well for me, and has resulted in all cookies saved on the hard level right to the end. Once its built, you can sit back, relax and let it play out.
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