easy multi kill hint. trap yourself in a 20x20 square of barrels(make sure you dont shoot and clear all of the demons beforehand) then when all of the zombies have gathered around the barrel shoot one. :) or, you can use those as an indestructible barrier, for use of mortars.
Cannon+Cannon+Laser+Rocket+rocket damage max on all of them, upgrade the range of the blue one the most and (if you didn't know) face away from the map and press d while selecting every turret there but the upgraded cannon. thats a big laser cluster bomb landmine (5 laser rockets per mine)
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