Iron Curtain achievement did not unlock

I did the den defense without using the canon and when I finished it the iron curtain achievement did not unlock. What exactly do I need to do to unlock that achievement?

 

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Last updated July 3, 2018 Views 0 Applies to:

I may be wrong but I think that the sync % has to be high too, I think I got 84% sync on it and got the achievement. Just try not to loose very many barricades and assassins and you should be alright.