another thing I missed: You have to make it so the sentries will die from one shot, because they will never die if you make it more. The way around this is to make the rounds become faster and faster, (I suggest starting at 3-5 seconds between each sentry and respective barrier switch) and make upgrades for damage and firing speed. You could also make a hard mode where the balloons fire back. also you can use this to make the sentries move.
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