Fairly good, but plays itself when you are maxed out. I found, although risky, that you can place an ion to the left of the center in the beginning of apocolypse, upgrade it, and place a bunker in the center. When you have enough money, place an ion above the bunker and upgrade it to level 3. The left turret will suffer and nearly die, but will survive and the top turret take most of the damage then all when at 3. Upgrade economics and have 4 level 3 turrets before building economic centers.
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