This is one of the most difficult Medium badges to get. Very frustrating to a lot of people that have only this achievement standing in their way of the Best of 2007 badge. Best advice I can say is to create a maze, but instead of upgrading dozens of towers once or twice, fully upgrade some key positions and leave the rest unupgraded just as the maze wall. Good luck!
Tactics: 1. When still upgrading tiers 1 and 2, upgrades for main cannon and fire support (Mines, Artillery Strike and Air Strike) are your best friends, unless you can afford a Laser Turret. 2. Get all Satellite Guns up and running by the time you start upgrading Tier 2. Mostly, it'll be close quarters with the swarms at this point. 3. Keep turrets as close as possible to your base - this strategy is for later levels in the game where the swarms tend to overwhelm a LOT. 4. Following tip #3, you can squeeze 5 barriers into each side’s perimeter. Each time one is destroyed, enemies will attack the next barrier that's close enough instead of a turret. Don't forget to replenish your barriers every now and then. 5. Fire support (except for Mines) is still deployed when you're researching or building. Technically, the game is 'paused' but the fire support still drops (and replenishes, if left long enough), leaving you with lesser enemies to deal with. + if this helped you get to the end
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