Cowfoot's guide is good in that protecting the population is a big part of the strategy, however, its not the most efficient. To improve your chances, instead of starting out with 5 ions with 5 bunkers behind them, try instead 5 ions with 3 bunkers. Each ion placed with one space between them. Place a bunker between the ions on the side and a bunker behind the center one. This'll save you 100B and give you the same protection.
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