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There are three defensive towers to choose from when fortifying your platforms:

  • Machine Gun
  • Artillery
  • Anti-Air

However, I'm almost done the second chapter of the game, and so far, my anti-air towers have always been 'greyed out' in the build menu. How can I access them?

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Have you had to fight any airborne enemies? In my experience with tower defense games, anti-air towers don't unlock until you need them. – murgatroid99 Jan 31 '12 at 4:03
Yes, I have. In the last level I completed, I had my first encounter with enemy helicopters, and the radio chatter made a point of saying 'build anti-air towers at all rear platforms!'. I have been fighting enemy aircraft since the first chapter though. Of note, is that if I do a 'quick game' from the menu, the anti-air towers are available. So it looks like maybe he was just talking about machine gun towers. Also, Oil Rush isn't a tower defense game. – GnomeSlice Jan 31 '12 at 17:48
What game is this for? – CyberSkull Aug 15 '12 at 0:40
up vote 2 down vote accepted

Patience young padawan :) They unlock a few missions later. In the meantime, upgraded bunkers work on helicopters. Also, in most missions upgrading them and researching the defense upgrade is essential even at the added expense. Otherwise, you constantly have to re-build them.

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What the hell are you talking about? Those are not helicopters, but flying aliens. And the "bunker" is called a "Gun Tower" – Sentry Apr 28 '13 at 10:24
I don't think this is actually about Defense Grid. I have no idea what they're talking about either. – super_aardvark Jun 15 '13 at 1:21

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